Harden note structure and evaluation with reviewed fixes

[Cat-blizzard]

The P0 structure and evaluation workflow needs to preserve valid lecture documents and report layout problems accurately. This integrates PR #9 with fixes for successful structure repair dropping H1, document framing duplicating suffixed global headings, and Markdown tables being split into multiple tables.

The final workflow preserves the original document title, shares heading matching between framing and validation, ignores fenced code in structure/evaluation/layout scans, and reports malformed table rows at their actual line. Document and section generation prompts now agree with the heading levels supplied by composition; affected prompt cache versions are updated. The existing body, source, image, coverage and truncation guards remain in place.

The branch retains PR #9's original commit and adds one reviewed fix commit. Merge with a merge commit so the original contribution remains in the main branch history.

Validation:

• Full test suite: 395 passed (41 additional regression cases).
• First-run smoke: passed without API calls.
• Original synthetic sample evaluation: 1/1 passed in local mode.
• Strict Markdown layout check: 0 errors, 0 warnings.
• Published tree hashes match all tested PR and fix files.

Real lecture LLM before/after comparisons, manual fact/source/figure checks, and actual DOCX/PDF export review remain pending; the synthetic run does not establish those quality claims.