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/propose.py

@@ -1514,6 +1514,22 @@ def build_plan(
         outline_marks=outline_marks,
         outline_rule=outline_rule,
     )
+    # A mark's description travels to the candidate whose span holds the mark's
+    # line -- the FIRST such mark, so a span that opens at zero (the front
+    # matter above the first section) still takes its own section's and no
+    # other. Only the NISO-STS reader sets one, so every other row's plan keeps
+    # its bytes: an absent key is the source saying nothing.
+    starts: list[int] = []
+    notes: list[str | None] = []
+    if outline_marks:
+        offsets = [0]
+        for line in text.splitlines(keepends=True):
+            offsets.append(offsets[-1] + len(line))
+        for mark in sorted(outline_marks, key=lambda item: item.line):
+            if mark.line < len(offsets) - 1:
+                starts.append(offsets[mark.line])
+                notes.append(mark.description)
+    cursor = 0
     for candidate in subdivide(text, candidates, max_segment_chars):
         entries.append(
             {
@@ -1550,6 +1566,12 @@ def build_plan(
                 "derived": _derived_names(candidate),
             }
         )
+        while cursor < len(starts) and starts[cursor] < candidate.start:
+            cursor += 1
+        if cursor < len(starts) and starts[cursor] < candidate.end:
+            description = notes[cursor]
+            if description is not None:
+                entries[-1]["description"] = description
     return {
         "version": "1",
         "source_sha256": hashlib.sha256(source_bytes).hexdigest(),