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>

README.md

@@ -640,7 +640,10 @@ bundle:
    the `sources` title from `<doc-number>` + `<year>`, then `<title-wrap>`,
    then the file name, whichever is the first that can be written verbatim.
    Stated more than once, or claimed by a second document in the same run, a
-   declared name is not used and the file name stays.
+   declared name is not used and the file name stays. Each titled section's
+   `description` is its own first spec point — the first `<p>` of its first
+   direct-child `<sec sec-type="spec">`, whole — and a section with none gets
+   no `description` at all; nothing is derived from the title.
    The drop directory is walked **recursively**, in sorted relative-path order:
    a file at any depth is ingested and records its path relative to the inbox
    root as its `source_file`, while dot-directories and a bundle directory