refactor(#1373): add canonical markdown-sectionizer seam (epic #1372 T0) (#1381)

* refactor(#1373): add markdown-sectionizer seam (ADR-1372 T0)

Establishes the canonical markdown-structure parsing seam per ADR-1372.
No existing parsers are modified; this is the foundational T0 tier only.

- docs/adr/1372-markdown-sectionizer-seam.md: Accepted ADR defining the
  seam interface, the tiered migration plan (T0-T7), and the prohibition
  enforcement approach (no-adhoc-markdown-parsing ESLint rule in T7).
- src/markdown-sectionizer.cts: Pure module, Node built-ins only.
  Exports: stripFencedCode (CommonMark-correct state machine ported from
  uat-predicate.cts _stripFencedBlocks, CRLF-safe, unterminatedFence
  signal), tokenizeHeadings (ATX headings outside fenced blocks),
  collectSections (line-by-line predicate-driven section collection),
  collectSection (single named section, levelBounded stop, optional
  stripFences), iterateBullets (dash/checkbox/numbered + continuation).
- tests/markdown-sectionizer.test.cjs: 54-test behavioral suite covering
  the parser QA matrix (LF/CRLF, Unicode headings, headings-inside-fences,
  unterminated fences, nested levels, all bullet markers, continuation
  lines, empty/non-string input) plus 4 fast-check property tests
  (idempotence, output shape, never-throws, length monotonicity).
- CONTEXT.md: Markdown Sectionizer glossary entry added (PR review gate).

Tests: 54 pass, 0 fail. Existing adr-parser + uat-passed tests: 22 pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* refactor(#1373): add extractTaggedBlocks + replaceSection to seam; register inventory

- src/markdown-sectionizer.cts: extend Section type with bodyStart/bodyEnd offsets;
  add extractTaggedBlocks(content, tagName) (inner text of <tag>…</tag> blocks,
  tagName regex-escaped, caller decides fence-stripping) and replaceSection(content,
  section, newBody) (pure character-offset splice for read-modify-write callers);
  update collectSections/collectSection to populate bodyStart/bodyEnd.
- tests/markdown-sectionizer.test.cjs: add 33 new behavioral tests for
  extractTaggedBlocks, replaceSection, and a DEFECT.GENERATIVE-FIX parity guard
  that asserts stripFencedCode and uat-predicate's _stripFencedBlocks agree on a
  shared 9-item corpus; documents the known 4-space-indent divergence.
- docs/adr/1372-markdown-sectionizer-seam.md: list extractTaggedBlocks and
  replaceSection in §"The seam".
- CONTEXT.md: update ### Markdown Sectionizer glossary entry with the two new exports.
- docs/INVENTORY.md: add markdown-sectionizer.cjs row (alphabetically between
  loop-resolver and milestone).
- docs/INVENTORY-MANIFEST.json: regenerated via gen-inventory-manifest --write.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(#1373): clear no-unsafe-assignment + unused-var lint in markdown-sectionizer

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(#1373): correct section offset/round-trip + CommonMark heading/fence edges; register eslint coverage

FIX 1 (CRITICAL): Enforce content.slice(bodyStart,bodyEnd) === body invariant in both
collectSection and collectSections. bodyEnd is now bodyStart + body.length instead of
the raw stop-line offset, eliminating the trailing-newline overcounting that caused
replaceSection to drop separator newlines (## A\nbody## B gluing bug).

FIX 2 (MED): tokenizeHeadings now accepts ≤3-space indent (CommonMark §4.5) and empty
ATX headings (## / ##   ), text=''. 4-space indent correctly excluded.

FIX 3 (MED): collectSection gains stopAtLevel option — stops at the next heading whose
level ≤ stopAtLevel, independent of the opener's level. Enables state.cts ## sections
that also stop at ### without abusing levelBounded.

FIX 4 (MED): Backtick fence opener info string must not contain a backtick (CommonMark).
Applied in both stripFencedCode and tokenizeHeadings fence state machines. Tilde fences
unaffected.

FIX 5 (LOW): "byte offset" → "character (string-index) offset" in HeadingToken / Section
doc comments.

FIX 6 (LOW): extractTaggedBlocks doc comment documents nested-tag non-support; test locks
the non-greedy close-at-first-</tag> behavior.

FIX 7: Add gsd-core/bin/lib/markdown-sectionizer.cjs to eslint.config.mjs ignores so
tests/551-eslint-bin-lib-coverage.test.cjs passes (3/3).

Tests: 107 pass / 0 fail (was 87; +20 new tests for FIX 1–4, 6).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(#1373): gitignore tsc-built markdown-sectionizer.cjs (ADR-457 build-at-publish)

The seam's compiled artifact must be a build-at-publish output like every other
src/*.cts->bin/lib/*.cjs module (decisions, core, state, ...), not a committed
file. Add it to the ADR-457 ignore list and untrack it; build:lib/CI regenerate it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

eslint.config.mjs

@@ -160,6 +160,8 @@ export default tseslint.config(
       'gsd-core/bin/lib/capability-writer.cjs',
       // issue #1355: tsc-generated runtime artifact — lint the src/teams-status.cts source.
       'gsd-core/bin/lib/teams-status.cjs',
+      // ADR-1372: tsc-generated runtime artifact — lint the src/markdown-sectionizer.cts source.
+      'gsd-core/bin/lib/markdown-sectionizer.cjs',
     ],
   },