docs(audit): align the stack counts and remove the stale AST claim in PROGRESS #88

Context

Follow-up from #84, which is itself a documentation-accuracy fix. Three inconsistencies remain:

1. Stack size has four values. The PR title says "ten-branch", the PR body says "twelve branches", docs/AUDIT.md (line 69) says "a linear stack of nine branches", and git log origin/main..HEAD on the merged head has 14 commits (12 layer commits plus two docs(progress) commits). Pick one canonical description (a table of layers plus a commit count is clearest) and use it everywhere.
2. The AST claim the PR set out to remove is still present. docs/PROGRESS.md line 24 (M4-T04) still reads "Strict SELECT AST validation, DDL/DML rejection, Markdown formatting". README.md was corrected to describe the lexer + dedicated role, and tests/unit/test_docs_accuracy.py guards only the README line.
3. Optional: guard the corrected PROGRESS wording in test_docs_accuracy.py so the claim cannot drift back.

Acceptance criteria

• One consistent branch/commit count across the PR description, docs/AUDIT.md and docs/PROGRESS.md (including the review follow-ups).
• Replace the AST wording in the M4-T04 row with the actual lexer/role description.
• Extend the docs-accuracy test to cover the PROGRESS wording.