feat(description): an STS section's description is its own first spec point

K3-19 c. The NISO-STS reader records, per titled <sec>, the FIRST <p> of its
FIRST direct-child <sec sec-type="spec"> as `OutlineMark.description`. The
plan entry carries it (`description`, only where the source has one, so every
other row's plan keeps its bytes), `parse_segmentation_plan` refuses an empty,
multi-line or non-string value, and the door writes it as the concept's
`description` after the gate has seen it: it is document text persisted
outside the body the gate screens, so it is kept only on the non-blocking
floor and as the sanitized text.

SPEC SS 4.1 makes `description` RECOMMENDED and sets no length, in SS 4.1,
SS 8 or SS 11. The limit is ours and structural -- one paragraph, whole --
because a cut inside it writes a sentence the source never wrote. Measured on
R761: 2 026 of 2 761 titled sections carry a direct-child spec point; the
first <p> runs 17 / 109 / 273 / 521 / 942 characters (min / median / p90 /
p99 / max). A section with none gets no key; nothing is derived from the
title. A stated `--frontmatter description=...` replaces it.

The extracted text does not move: the description is read beside it.

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

src/llm_ingestion_okf/segmentation.py

@@ -157,6 +157,11 @@ class SegmentEntry:
     derived: frozenset[str] = field(default_factory=frozenset)
     anchor: SegmentAnchor | None = None
     adjudication: SegmentVerdict | None = None
+    #: The SOURCE's own one-line summary of this segment, where it declares one
+    #: (NISO-STS: the section's first spec point). Carried in the plan beside
+    #: `title` for the same reason `title` is: a plan is the record a rebuild
+    #: replays, and the door writes it as the concept's `description`.
+    description: str | None = None
 
 
 @dataclass(frozen=True)
@@ -363,6 +368,20 @@ def _parse_entry(payload: Any, *, position: int) -> SegmentEntry:
             f"{where} field 'parent_id' must be a non-empty string or absent, got {parent_id!r}",
             code="segmentation_plan_invalid",
         )
+    description = payload.get("description")
+    if description is not None and (
+        not isinstance(description, str)
+        or not description
+        or "\n" in description
+        or "\r" in description
+    ):
+        # Written into line-oriented frontmatter, so a line break would inject
+        # a key and an empty value would state a summary that says nothing.
+        raise SegmentationError(
+            f"{where} field 'description' must be a non-empty single-line string or "
+            f"absent, got {description!r}",
+            code="segmentation_plan_invalid",
+        )
     return SegmentEntry(
         segment_id=segment_id,
         path=normalize_segment_path(payload["path"], where=where),
@@ -374,6 +393,7 @@ def _parse_entry(payload: Any, *, position: int) -> SegmentEntry:
         derived=_parse_derived(payload.get("derived", ()), where=where),
         anchor=_parse_anchor(payload.get("anchor"), where=where),
         adjudication=_parse_verdict(payload.get("adjudication"), where=where),
+        description=description,
     )