feat(propose): --shell-parent points a heading-only section at the ancestor holding its text

A plan entry whose span holds its heading alone gets `parent_id` naming the
nearest preceding entry at a smaller level whose own span holds text, passing
over an ancestor that is empty too; the door writes the existing `parent:`
key. Nothing is copied and no boundary moves. The rule reads the plan's level
and order, never the row. Off by default: `okf consume` reads no `parent`
key, so no payload ranks differently, while the flag moves the bytes of every
bundle holding a heading-only section.

Measured before building on one 2 761-concept process code: 710 concepts are
heading-only; the level route names the ancestor the document's own <sec>
nesting names on 708 of 710 (two sit at depth 7, clipped to 6, and point one
level too high), where reading section numbers gets 686 (`12` begins with
`1`); 35 have no ancestor holding text and get none.

The red test expected only concept files to move. The index is a projection
of the frontmatter and shows the key as a facet, so the test now holds both;
the facet renders a segment id as unresolved (`p1?`), because `structure`
reads `parent` as a document number -- named in README, CLAUDE.md and the
CHANGELOG, not repaired here.

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

CHANGELOG.md

@@ -7,6 +7,24 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- **`okf build --shell-parent` (K3-20), off by default.** A concept whose body
+  is its heading alone gets `parent:` naming the `segment_id` of the nearest
+  ancestor that holds text: the nearest preceding plan entry at a smaller
+  level, passing over an ancestor that is empty too. Nothing is copied and no
+  boundary moves. It reads the plan's level and order, never the row. Measured
+  on one process code, 710 of 2 761 concepts are heading-only; the route names
+  the ancestor the document's own nesting names on 708 of them (two sit at
+  depth 7, which a markdown heading clips to 6), where reading section numbers
+  gets 686.
+  - **Off, by measurement:** `okf consume` reads no `parent` key, so no payload
+    ranks differently, while the flag moves the bytes of every bundle holding
+    a heading-only section.
+  - **Known cost:** the index projects `parent` as a document NUMBER, so a
+    segment id always renders unresolved there (`parent: p1?`). The same key
+    already carries both meanings for an adjudicator's declared parent.
+
 ### Fixed
 
 - **A directory every concept id shares no longer ranks the concepts