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>

src/llm_ingestion_okf/segmentation.py

@@ -45,6 +45,18 @@ from typing import Any
 from .errors import SegmentationError
 from .materialize import reduce_to_id_grammar
 
+
+def heading_only(text: str) -> bool:
+    """`True` when no line of `text` holds anything but a markdown heading.
+
+    ONE predicate for two readers that must agree: the proposer gives such a
+    span a parent (`propose._link_shells`), and the door writes a link into
+    exactly those bodies. Both read the same window on the same extracted text,
+    so a second copy of the rule could only ever disagree with the first.
+    """
+    return not any(line.strip() and not line.startswith("#") for line in text.split("\n"))
+
+
 #: The top-level keys a plan payload must carry. Every one is required: a plan
 #: missing its extractor identity would still parse, and would then be replayed
 #: against an extraction nobody checked it against.