TDD 0055: markdown section/bullet parser unification (md.sh)

[cahenesy]

Design PR (gate = your merge). One new TDD, Status: draft. The deferred md-section follow-up to 0049, from the consolidated scripts audit.

What & why

One new scripts/lib/md.sh single-sources the markdown-structure parsers and folds the bugs living in them. PRD unchanged (0aa1e28); gap-closure, same category as 0048–0054.

• md_section_body <file> <heading> — fence-aware (``` and ~~~) in-section walk, rc-checked; callers layer their own predicate.
• md_bullet_path_of_line <bullet-line> — the shared per-bullet path op (0049's annotation-robust algorithm), reused by md_bullet_path and the Expected-diff callers (count/exception stay caller-side).
• md_bullet_path <file> <heading> [mode] — md_section_body | /^- / | md_bullet_path_of_line; rc-propagating.

Routes 4 section-scan sites (plan-classifier, tdd-lint ×2, gates) + the ## Expected diff size path extraction + 0049's tl_extract_touched_paths through md.sh.

Folds: reuse #11 (plan-classifier.sh was the non-fence-aware copy → classifier-misroute), #12 (the un-cross-checked Expected-diff twin); bugs A21 (~~~ fences ignored), A23 (count vs extract anchor mismatch), A4 (gawk fatal exit 2 collides with the missing-section exit 2), L-005 (tl_extract_touched_paths swallows awk's exit code → spurious structural-finding(a)). The L-005 rc now propagates end-to-end through the delegation wrappers (split-declared captures dodge the local x="$(…)" rc mask) to _rework_pre_pass, which routes a parse failure away from structural-finding(a).

Build ordering (declared dependency)

Builds LAST: after 0049's impl (PR #142, which lands touched-files.sh) and after the 0050–0054 stack (shares gates.sh/tdd-lint.sh). Nothing stacks on 0055.

Design gate

• Mechanical: tl_lint rc=0; --bounds rc=0; body 265/500; 8 files, no exception.
• Independent critique (design-reviewer, sonnet): first pass DESIGN_REVIEW: BLOCK on two real gaps it caught — (1) md_bullet_path couldn't serve the count/exception Expected-diff callers; (2) the L-005 rc would be masked by local x="$(…)" before reaching the membership check. Both fixed (shared md_bullet_path_of_line; explicit split-declared rc propagation) plus the minors (count fence walk migrated; A4 sequencing; advisory _coverage_inscope_reqs carve-out). Re-review: DESIGN_REVIEW: PASS.
• ADR evaluation: zero new ADRs; ADR 0006 governs and is respected.

Open assumptions & waivers

• authoring against unmerged 0049 interface — resolved: 0049's design (TDD 0049: Touched-files extractor — single source of truth + annotation-robust #140) locks the tl_extract_touched_paths interface; 0055 declares the 0049-impl build-dependency and builds after build/20260610 202343/0049 touched files extractor unify and harden #142.
• md_section_body must preserve each caller's in-section predicate — resolved: it emits raw fence-aware in-section lines; each caller keeps its own classify/row/column logic.
• honoring ~~~ fences changes behavior — resolved: the ```-only parsers mis-handle ~~~ (the bug); honoring it is the fix, regression-pinned.
• L-005 awk-rc check semantics — resolved: non-zero awk → rc 2 + stderr (never silent empty), so parse-fail is distinguishable from a genuinely-empty section.
• touched-files delegate must not change output / break the 0049 cross-check — resolved: tl_extract_touched_paths keeps name/signature/output; the 0049 3-way agreement test still passes (Verification pins it).

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