test(okf): pin the two boundary classes the new carriers introduced

Probed after 1.3.0 landed, on advisor challenge. Both came back clean --
these pin them so they cannot regress silently.

1. A `- ` item whose text begins with a YAML indicator AND carries a `": "`
   could in principle route into the block-mapping path with a
   half-validated key instead of into `_reject_dangerous_value`. It does
   not: `&anchor id` fails `_KEY_RE` on the key side, so the item falls
   through intact and raises on the indicator. One row each for & * ! %
   backtick, plus the merge key inside an entry.

2. `_consume_block_mapping` ends at a blank line and at column zero, and
   hands an index back to `_consume_block_list`, which hands one back to
   `_parse_flat`. A line orphaned by that boundary -- a `resource:` left
   over from a mapping that closed early -- must RAISE, not be dropped. A
   pointer that vanishes rather than failing is this repo's failure class,
   so it gets a test rather than a probe. Plus the return-index contract:
   a top-level key after a multi-entry list is neither swallowed nor
   re-read as an item.

868 green (was 859), 130/130 classes. Test-only; no source change.

tests/test_okf.py

@@ -1092,3 +1092,43 @@ def test_the_producer_golden_now_passes_the_gate():
     result = import_bundle({"datasets/sales.md": doc})
     assert result.disposition is Disposition.WARN
     assert result.concepts[0].error is None
+
+
+@pytest.mark.parametrize("cid,fm", [
+    # A `- ` item whose text begins with a YAML indicator AND carries a `": "`
+    # must fall through to the scalar predicates, not into the block-mapping
+    # route with a half-validated key. Each of these fails `_KEY_RE` on the key
+    # side, so `_reject_dangerous_value` gets the item intact.
+    ("anchor", "sources:\n  - &anchor id: a\n"),
+    ("alias", "sources:\n  - *anchor id: a\n"),
+    ("tag", "sources:\n  - !!str id: a\n"),
+    ("directive", "sources:\n  - %YAML id: a\n"),
+    ("reserved", "sources:\n  - `x id: a\n"),
+    ("merge key", "sources:\n  - id: a\n    <<: *base\n"),
+])
+def test_an_indicator_in_a_block_item_is_refused_before_the_mapping_route(cid, fm):
+    with pytest.raises(OKFFrontmatterError):
+        parse_frontmatter(f"---\ntype: t\n{fm}---\n\nbody\n")
+
+
+@pytest.mark.parametrize("cid,fm", [
+    # The block mapping ends at a blank line and at a line in column zero. Both
+    # hand control back with an index that must not skip or re-read a line: a
+    # dangling `resource:` line left over from a mapping that closed early must
+    # RAISE, never be silently dropped -- a pointer that vanishes rather than
+    # failing is exactly this repo's failure class.
+    ("blank line inside the mapping", "sources:\n  - id: a\n\n    resource: b\n"),
+    ("top-level key interleaved", "sources:\n  - id: a\ntags: x\n    resource: b\n"),
+])
+def test_a_line_orphaned_by_the_mapping_boundary_raises_rather_than_vanishing(cid, fm):
+    with pytest.raises(OKFFrontmatterError):
+        parse_frontmatter(f"---\ntype: t\n{fm}---\n\nbody\n")
+
+
+def test_the_block_list_hands_back_an_index_that_resumes_at_the_next_key():
+    # The return-index contract: a top-level key following a multi-entry block
+    # list is neither swallowed by the list nor re-read as a list item.
+    fm, _ = parse_frontmatter(
+        "---\ntype: t\nsources:\n  - id: a\n    resource: b\n  - id: c\ntitle: T\n---\n\nbody\n")
+    assert fm == {"type": "t", "sources": [{"id": "a", "resource": "b"}, {"id": "c"}],
+                  "title": "T"}