fix(frontmatter): a nested key must not substitute for a top-level one

The line-oriented frontmatter grammar exists in three copies, each with the
duplication documented at its site: `materialize` reads a path, `structure`
needs a character offset, `profiles` returns body lines. All three keyed on
`key.strip()`, which discards the indentation that is the only thing telling
a nested key from a top-level one. An indented `title:` under a `sources:`
block therefore landed in the same flat namespace as the document's own
`title:` and, arriving later, won.

The failure is substitution, not omission. A dropped value is visible to
whoever reads the concept; a substituted one is not -- the document carries a
title that looks entirely right and belongs to something else. Because
`number` derives from `title` and `parent` derives from `number`, one
substitution walks the hierarchy. Measured, not inferred: a document titled
`N100.2` with a nested source titled `N200.7` came back as N200.7 with parent
N200 instead of N100.2 with parent N100.

Measured incidence across the two corpora, denominators stated:
`_okf-canonical` @ ad30107, 54 documents with parsable frontmatter, 49 carry
a nested key colliding with a top-level name (90.7%); `_okf-upstream` @
9a15b13, 66 documents, 58 collide (87.9%). The colliding key is `title`, and
often `resource` with it -- in `acme_retail/tables/orders.md` the concept's
own BigQuery resource pointer was replaced by a nested one. This is a fix
that clears observed damage, not a hardening without a witness.

The fix refuses indented lines; it does not read them. Block form stays
unreadable -- `sources` and `verified` still come back empty -- so D4's
flow-form emission rule is untouched and the structured reader is still D1b.
Two characterization tests that pinned the old behaviour now pin the new: the
block-list family still DROPS its value, and only the key-space pollution is
gone. That family is not otherwise addressed here.

Test first, red before the code was touched, with known-positive controls for
all three parsers so that a parser returning nothing could not pass.

Order: 20260830T000740Z-4733930312-from-.claude

Co-Authored-By: Claude <claude-opus-5>

tests/test_attested_computation.py

@@ -160,20 +160,26 @@ def test_the_contract_fields_round_trip_in_their_flow_form(tmp_path: Path) -> No
     assert parse_frontmatter(path) == values
 
 
-def test_two_nested_block_mappings_sharing_a_key_collide_in_the_scalar_parser(
+def test_two_nested_block_mappings_sharing_a_key_are_refused_not_flattened(
     tmp_path: Path,
 ) -> None:
-    """Measured 2026-07-31, and the reason D4 stops at the flow form.
+    """Measured 2026-07-31, re-measured 2026-08-31, and still the reason D4
+    stops at the flow form.
 
     §10.2 presents `executor` and `attester` as nested BLOCK mappings, and both
-    carry a `resource`. The line-oriented parser has no indentation model, so it
-    flattens them into one namespace where the second `resource` overwrites the
-    first: `executor.resource` is lost and `attester.resource` surfaces as a
-    top-level key. No error is raised.
+    carry a `resource`. Neither is READABLE here -- both come back empty, which
+    is exactly why D4 pins emission to the flow form and why reading this shape
+    is D1b's work, not this parser's.
 
-    Pinned rather than fixed. Reading this form needs the structured reader
-    (D1b), and a half-reader that silently drops half a contract is worse than
-    one that never claimed to read it.
+    What the parser no longer does is FLATTEN them. Before order
+    `...4733930312` it had no indentation model at all: both `resource` lines
+    landed in the document's own namespace, the second overwrote the first, and
+    `attester.resource` surfaced as a top-level `resource` that no document
+    declared -- silently, with no error. A dropped value is visible to whoever
+    reads the concept; a substituted one is not.
+
+    So the contract is still half-read, and that is deliberate. It is now
+    half-read by REFUSAL rather than by substitution.
     """
     path = tmp_path / "block-form.md"
     path.write_text(
@@ -192,8 +198,13 @@ def test_two_nested_block_mappings_sharing_a_key_collide_in_the_scalar_parser(
 
     assert parsed["executor"] == ""
     assert parsed["attester"] == ""
-    assert parsed["resource"] == "attesters/revenue.py"
+    # The substitution is gone: neither nested `resource` reaches the namespace.
+    assert "resource" not in parsed
+    assert "receipt" not in parsed
+    assert set(parsed) == {"type", "runtime", "executor", "attester"}
+    # Still unreadable, as D4 requires -- neither value survives anywhere.
     assert "skills/run-on-bq.md" not in parsed.values()
+    assert "attesters/revenue.py" not in parsed.values()
 
 
 # --- door C surfaces the §10 pointers it imports ---------------------------

tests/test_frontmatter_nesting.py

@@ -0,0 +1,172 @@
+"""A nested frontmatter key must never substitute for a top-level one.
+
+This library's line-oriented frontmatter grammar exists in three copies, by
+design and with the duplication documented at each site: `materialize`
+reads a path, `structure` needs a character offset, `profiles` returns body
+lines. All three split a line on its first colon and key the result on
+`key.strip()` — which discards the indentation that is the ONLY thing
+distinguishing a nested key from a top-level one. An indented `title:` under
+a `sources:` block therefore lands in the same flat namespace as the
+document's own `title:` and, arriving later, wins.
+
+The failure this closes is not a dropped value but a SUBSTITUTED one. A
+missing title is visible to whoever reads the concept; a substituted title
+is not — the document carries a title that looks entirely right and belongs
+to something else. And because `number` derives from `title` and `parent`
+derives from `number`, one substitution walks the whole hierarchy. That is
+`P4` (a concept ID is a path, and a path is a promise) broken in the one
+property no later run can repair.
+
+Scope: this is the SUBSTITUTION defect only. The block-list family — a
+`- item` line carrying no colon and being dropped — is the same family and a
+different defect, and is deliberately not addressed here. Nor is this a
+structured YAML reader; that is D1b. Flow form stays the emission rule, and
+the round-trip tests below pin that the fix does not disturb it.
+"""
+
+from pathlib import Path
+
+from llm_ingestion_okf.materialize import parse_frontmatter
+from llm_ingestion_okf.profiles import _split_frontmatter as _profiles_split
+from llm_ingestion_okf.structure import _split_frontmatter as _structure_split
+from llm_ingestion_okf.structure import derive_document_structure
+
+# A `sources:` block whose entry carries its own `title:`. The top-level title
+# and the nested one are both well-formed document titles, and both bear a
+# number — which is what lets the test measure `number` and `parent` moving
+# rather than only asserting that they could.
+NESTED = """\
+---
+title: N100.2 Kryss og avkjoersler
+generated: true
+source_file: vegnormal.md
+sources:
+  - resource: https://example.test/bruprosjektering.pdf
+    title: N200.7 Bruprosjektering
+---
+
+# Kryss og avkjoersler
+
+Body text.
+"""
+
+# The same document with the nested block removed. Nothing else differs.
+FLAT = """\
+---
+title: N100.2 Kryss og avkjoersler
+generated: true
+source_file: vegnormal.md
+---
+
+# Kryss og avkjoersler
+
+Body text.
+"""
+
+
+# --- known-positive controls ---------------------------------------------
+#
+# Without these, a parser that returned nothing at all would pass every
+# assertion below: "the nested title did not win" is satisfied by a parser
+# that reads no title whatsoever. Each control proves the SAME parser, on the
+# SAME shape, actually finds the top-level key when no nested key competes.
+
+
+def test_control_materialize_reads_top_level_title(tmp_path: Path) -> None:
+    path = tmp_path / "vegnormal.md"
+    path.write_text(FLAT, encoding="utf-8")
+    assert parse_frontmatter(path)["title"] == "N100.2 Kryss og avkjoersler"
+
+
+def test_control_structure_reads_top_level_title() -> None:
+    declared, offset = _structure_split(FLAT)
+    assert declared["title"] == "N100.2 Kryss og avkjoersler"
+    assert offset > 0
+
+
+def test_control_profiles_reads_top_level_title() -> None:
+    head, _body = _profiles_split(FLAT)
+    assert head["title"] == "N100.2 Kryss og avkjoersler"
+
+
+def test_control_derivation_reads_top_level_title() -> None:
+    structure = derive_document_structure(FLAT, source_file="vegnormal.md")
+    assert structure.title == "N100.2 Kryss og avkjoersler"
+    assert structure.number == "N100.2"
+    assert structure.parent_number == "N100"
+
+
+# --- the defect, once per parser copy ------------------------------------
+
+
+def test_nested_title_does_not_substitute_in_materialize(tmp_path: Path) -> None:
+    path = tmp_path / "vegnormal.md"
+    path.write_text(NESTED, encoding="utf-8")
+    assert parse_frontmatter(path)["title"] == "N100.2 Kryss og avkjoersler"
+
+
+def test_nested_title_does_not_substitute_in_structure() -> None:
+    declared, _offset = _structure_split(NESTED)
+    assert declared["title"] == "N100.2 Kryss og avkjoersler"
+
+
+def test_nested_title_does_not_substitute_in_profiles() -> None:
+    head, _body = _profiles_split(NESTED)
+    assert head["title"] == "N100.2 Kryss og avkjoersler"
+
+
+# --- the propagation the order asks to be MEASURED, not assumed ----------
+
+
+def test_substituted_title_moves_number_and_parent() -> None:
+    """`title` -> `number` -> `parent`, all three, on a real derivation.
+
+    The source filename carries no number, so `number` falls through to the
+    title — which is precisely the path a substituted title travels. This
+    asserts the whole chain rather than the first link, because the order's
+    question was whether `number` and `parent` move in PRACTICE or only in
+    theory.
+    """
+    structure = derive_document_structure(NESTED, source_file="vegnormal.md")
+    assert structure.title == "N100.2 Kryss og avkjoersler"
+    assert structure.number == "N100.2"
+    assert structure.parent_number == "N100"
+
+
+def test_nested_key_does_not_invent_a_top_level_field() -> None:
+    """A nested `resource:` must not appear as a top-level `resource`.
+
+    The substitution has a quieter twin: a nested key with NO top-level
+    counterpart is not overwriting anything, it is fabricating a field the
+    document never declared. `derive_document_structure` exposes `declared`
+    directly, so this pins the namespace itself and not one lucky key.
+    """
+    structure = derive_document_structure(NESTED, source_file="vegnormal.md")
+    assert "resource" not in structure.declared
+    assert set(structure.declared) == {"title", "generated", "source_file", "sources"}
+
+
+# --- the emission rule the fix must not disturb --------------------------
+
+
+def test_flow_form_still_round_trips(tmp_path: Path) -> None:
+    """Flow form is a single top-level line and must be untouched by the fix.
+
+    This library pins its own emission to flow form precisely because the
+    line-oriented parser round-trips it as one opaque string. A fix that
+    keyed on anything other than indentation could break that, so it is
+    pinned here rather than assumed.
+    """
+    flow = (
+        "---\n"
+        "title: N100.2 Kryss og avkjoersler\n"
+        "generated: { by: process:okf-ingest, at: 2026-08-31T00:00:00Z }\n"
+        "sources: [ a.pdf, b.pdf ]\n"
+        "---\n\nBody.\n"
+    )
+    path = tmp_path / "flow.md"
+    path.write_text(flow, encoding="utf-8")
+    parsed = parse_frontmatter(path)
+    assert parsed["generated"] == "{ by: process:okf-ingest, at: 2026-08-31T00:00:00Z }"
+    assert parsed["sources"] == "[ a.pdf, b.pdf ]"
+    assert parsed["title"] == "N100.2 Kryss og avkjoersler"

tests/test_okf_v0_2_characterization.py

@@ -65,15 +65,23 @@ def test_an_inline_flow_mapping_survives_the_scalar_parser_verbatim(tmp_path: Pa
     assert parse_frontmatter(path) == values
 
 
-def test_a_block_list_pollutes_the_scalar_parsers_key_space(tmp_path: Path) -> None:
+def test_a_block_list_is_dropped_without_polluting_the_key_space(tmp_path: Path) -> None:
     """The measured reason `sources` is emitted as an inline flow sequence.
 
-    Upstream's canonical `sources` is a block list of multi-key mappings. Read
-    through this line-oriented parser, each item line becomes a KEY: the list
-    disappears and `- id` / `resource` appear as frontmatter keys that no
-    document declared. `_is_ingest_owned` reads through this same parser, so
-    emitting the block form would have forced the gate and the parser to be
-    hardened in one step.
+    Upstream's canonical `sources` is a block list of multi-key mappings, and
+    this line-oriented parser still cannot READ one: the list value comes back
+    empty. That is unchanged, and it remains the whole reason this library
+    emits the flow form -- a value it can write is a value it can read back.
+
+    What changed (2026-08-31, order `...4733930312`) is the second, quieter
+    half. The item lines no longer become KEYS. `- id` and `resource` are
+    indented, so they belong to the block above them and are refused rather
+    than flattened into the document's namespace. The distinction matters
+    because `_is_ingest_owned` reads through this same parser: a fabricated
+    top-level key is a fact about the document that no document declared.
+
+    Refusing is not parsing. The block form stays unreadable; it is now
+    unreadable LOUDLY rather than by substitution. Reading it is D1b.
     """
     path = tmp_path / "concept.md"
     path.write_bytes(
@@ -84,8 +92,9 @@ def test_a_block_list_pollutes_the_scalar_parsers_key_space(tmp_path: Path) -> N
     parsed = parse_frontmatter(path)
 
     assert parsed["sources"] == ""
-    assert parsed["- id"] == "margin-standard"
-    assert parsed["resource"] == "policies/margin-standard.md"
+    assert "- id" not in parsed
+    assert "resource" not in parsed
+    assert set(parsed) == {"type", "sources"}
 
 
 def _stamped_concept(generated: str) -> bytes: