feat(doc-index): infer real section granularity, hash sections for staleness

[tkcoding]

Stacked on constructorfabric#108 (constructorfabric#108)

This branches from jit-retrieval-doc-index, the branch behind constructorfabric#108 — it needs doc_index.py/toc.py to exist first, so it's opened here (within this fork) rather than against upstream directly, since GitHub won't let a cross-repo PR base on a branch that only exists on the fork. Retarget to constructorfabric/studio:main once constructorfabric#108 merges.

Summary

Resolves two of constructorfabric#108's follow-up open questions (findings doc §13b/§13c).

• infer_section_level(): picks which heading level represents one real retrievable section, using the level's frequency as the signal. A document's real recurring structure (its chapters) shows up as the level used most often; an occasional heading at an anomalous level — exactly what PDF-to-Markdown conversion produces, since it assigns levels by font-size heuristics, not semantic depth — is rare precisely because it's noise, not structure. Levels used only once are excluded as candidates outright.

  This is a direct, verified fix for a real failure found building feat(toc,doc-index): add JIT-retrieval readiness checks and cached doc index constructorfabric/studio#108: a real PDF-converted document put all 8 of its actual chapters on H5 and a single stray subsection on H3. Treating H3 as "the" section level (or any fixed level) turned the entire back half of the document into one fake 6,601-line "section". Re-run against that same document with this change: 12 correctly-sized real sections, not one.

• retrieval_sections (new field on the built index): headings grouped at exactly the inferred level — off-level stray headings stay inside whichever section they geographically fall under, rather than splitting one apart — each with a SHA-256 hash of its own text.

• diff_stale_sections(): compares a file's current content against its last cached build at this granularity and reports which sections actually changed, matched by position (not heading text — duplicate titles are real, see toc-heading-duplicate). This is the piece a future caller needs to re-summarize only what changed instead of the whole document — the whole-file etag from feat(toc,doc-index): add JIT-retrieval readiness checks and cached doc index constructorfabric/studio#108 can only say "something changed", not "what".

Existing sections/annotate_section_summary/etag behavior is untouched.

Test plan

• pytest tests/test_doc_index.py tests/test_toc.py — 156 passed, 100% coverage on touched files
• Full suite — 4815 passed; the 12 failures present are the same pre-existing macOS-local/flaky ones seen on feat(toc,doc-index): add JIT-retrieval readiness checks and cached doc index constructorfabric/studio#108, none in files touched here
• pylint / vulture clean; cfs validate 0 errors; spec-coverage thresholds met
• infer_section_level re-run for real against the actual PDF-converted document that originally exposed the bug (see commit message)