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>

src/llm_ingestion_okf/cli.py

@@ -172,6 +172,13 @@ DEFAULT_FIRST_SPAN_FROM_ZERO = True
 #: rule closes each span against the next SURVIVOR. It adds no boundary, so the
 #: concept count is unchanged and the cost to a consumer is a re-run.
 DEFAULT_CLOSE_SPAN_GAPS = True
+#: Round 20 (K3-20). A section whose body is its heading alone gets `parent:`
+#: naming the nearest ancestor that holds text (`propose._link_shells`). NOT a
+#: segmentation rule: it adds no boundary and copies no text, and the concept
+#: count is unchanged. OFF, and the default is a measurement of what it buys:
+#: `okf consume` reads no `parent` key, so no payload ranks differently with
+#: it, while it moves the bytes of every bundle holding a heading-only section.
+DEFAULT_SHELL_PARENT = False
 #: Round 9. Clause 1 asked whether a title ENDED in a number, which is a
 #: question about the number rather than about the title: a drawing's dimension
 #: chain, a P&ID's schematic labels, a door schedule and a borehole log's
@@ -353,6 +360,7 @@ def _propose_plans(
     pdf_headings_reserve: bool = False,
     ocr: bool = False,
     pdf_outline: bool = DEFAULT_PDF_OUTLINE,
+    shell_parent: bool = DEFAULT_SHELL_PARENT,
 ) -> tuple[int, int, int]:
     """Propose a plan per dropped file. Returns (written, nothing, failed).
 
@@ -393,6 +401,7 @@ def _propose_plans(
                 pdf_headings_reserve=pdf_headings_reserve,
                 ocr=ocr,
                 pdf_outline=pdf_outline,
+                shell_parent=shell_parent,
             )
         except ProposerError as exc:
             print(f"{CLI_ID}: {relative.as_posix()}: {exc}", file=sys.stderr)
@@ -431,6 +440,7 @@ def build(
     pdf_headings_reserve: bool = DEFAULT_PDF_HEADINGS_RESERVE,
     ocr: bool = DEFAULT_OCR,
     pdf_outline: bool = DEFAULT_PDF_OUTLINE,
+    shell_parent: bool = DEFAULT_SHELL_PARENT,
     frontmatter: Mapping[str, str] | None = None,
 ) -> CorpusReport:
     """Folder in, bundle out. The whole command, minus argument parsing.
@@ -511,6 +521,7 @@ def build(
             pdf_headings_reserve=pdf_headings_reserve,
             ocr=ocr,
             pdf_outline=pdf_outline,
+            shell_parent=shell_parent,
         )
         print(
             f"{CLI_ID}: proposed {written} plan(s); {nothing} document(s) with no boundary; "
@@ -830,6 +841,26 @@ def parse_args(argv: list[str] | None) -> argparse.Namespace:
             "pre-2026-09-11 spans byte for byte"
         ),
     )
+    build_parser.add_argument(
+        "--shell-parent",
+        action="store_true",
+        default=DEFAULT_SHELL_PARENT,
+        help=(
+            "Give a concept whose body is its heading alone a `parent:` naming "
+            "the nearest ancestor that holds text, by the plan's level and "
+            "order, passing over an empty ancestor. Nothing is copied and no "
+            "boundary moves. Measured on one standard: 710 of 2 761 concepts "
+            "are heading-only, and the route names the ancestor its own "
+            "nesting names on 708 of them. OFF: `okf consume` does not read "
+            "the key"
+        ),
+    )
+    build_parser.add_argument(
+        "--no-shell-parent",
+        action="store_false",
+        dest="shell_parent",
+        help="The explicit form of the default",
+    )
     build_parser.add_argument(
         "--contents-name",
         action="store_true",
@@ -982,6 +1013,7 @@ def main(argv: list[str] | None = None) -> int:
             pdf_headings_reserve=args.pdf_headings == "font-reserve",
             ocr=args.ocr,
             pdf_outline=args.pdf_outline,
+            shell_parent=args.shell_parent,
             frontmatter=frontmatter_from_flags(args.frontmatter or ()),
         )
     except (IngestError, OSError, ValueError) as exc: