feat(consume): parent reaches the reader -- excerpt field, body link, checker rule

K3-21 A. `okf consume` resolves a concept's `parent:` pointer -- a
`segment_id`, unique only inside one document's plan -- among the concepts
sharing its `source_file` (`consume.link_parents`, one pass, no file opened
again) and an excerpt carries `parent: { concept_id, title }`. Conditional
like `req_number`: a concept with no `parent` key moves no byte. A pointer
that lands nowhere is named `parent_unresolved: true`, never dropped.

The door writes ONE line into a heading-only body whose entry has a parent:
`Enclosing section: [<title>](/<bundle-relative path>)` (SPEC SS 5.1 lineage
through links, SS 6.1 the recommended absolute form and the kind in the
prose). Only such a body, so the segmented goldens' declared parents -- bodies
holding text -- are untouched. Appended AFTER structure derivation and
screened on its own (`_screened`, the `description` rule): read as body text
the link was derived into a second, unresolved `references` edge, measured on
the fixture. `segmentation.heading_only` is the one predicate the proposer and
the door share.

`okf check` gains its seventeenth rule, `parent_unfollowable`: a `parent`
that is not a concept_id and title, names its own excerpt, or names a concept
in neither `excerpts` nor `withheld` (together every considered concept).
Contract SS 8 point 6 added, the figure carries `parent`, and "additional
members are not read by the checker" now says the checker reads only the
members SS 8 names. The template tells the reader what `parent` is and that
SS 2.2 lets it read that one concept; `skill.CONDITIONAL_FIELDS` gains
`parent`. README and CLAUDE.md say what consume now reads.

Moved on purpose, each named: the SS 7.4 known-positive IS the contract
document, so `budget.known_positive` moves in every payload (13 238 / 12 893
/ 345 -> 14 455 / 14 083 / 372); `skills/okf-consume/` regenerated from the
segmented golden, whose plan declares s1 and s2 under s0 -- its example
payload now carries both parents; `test_bundle_identity` 16 -> 17 rules;
`test_shell_parent`'s byte test also accounts for the link line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_shell_parent.py

@@ -168,16 +168,22 @@ def test_a_build_without_the_flag_is_the_build_with_the_opt_out(tmp_path: Path)
     assert files == sorted(p.relative_to(opted_out) for p in opted_out.rglob("*") if p.is_file())
     for relative in files:
         assert (default / relative).read_bytes() == (opted_out / relative).read_bytes()
-    # The flag adds one `parent:` line to each shell with an ancestor, and the
-    # index -- a projection of the frontmatter -- shows the same key as a facet
-    # on that shell's entry. Nothing else moves.
-    added = 0
+    # The flag adds one `parent:` line to each shell with an ancestor, and --
+    # since K3-21 -- one link line to that shell's body
+    # (`tests/test_parent_reaches_reader.py`). The index, a projection of the
+    # frontmatter, shows the same key as a facet on that shell's entry. Nothing
+    # else moves.
+    added = linked = 0
     for relative in files:
         before = (default / relative).read_text(encoding="utf-8").split("\n")
-        after = (flagged / relative).read_text(encoding="utf-8").split("\n")
+        text = (flagged / relative).read_text(encoding="utf-8")
         if relative.name == "index.md":
+            after = text.split("\n")
             assert [re.sub(r"parent: p\d+\?; ", "", line) for line in after] == before
             continue
+        text, links = re.subn(r"\n\nEnclosing section: \[[^\]]+\]\(/[^)\s]+\)\n\Z", "\n", text)
+        linked += links
+        after = text.split("\n")
         assert [line for line in after if not line.startswith("parent: ")] == before
         added += sum(1 for line in after if line.startswith("parent: "))
-    assert added == sum(1 for ancestor in EXPECTED.values() if ancestor is not None)
+    assert added == linked == sum(1 for ancestor in EXPECTED.values() if ancestor is not None)