T1.c2 — timeline --check ignores latest row's Commits cell

Change: tools/timeline.py main() --check: regex masks | <rows[-1] date> | <n> | Commits cell to # on both regenerated and on-disk text before comparing (md; svg carries no per-row count, mask is a no-op there). Docstring line for --check notes why. Write mode unchanged.
Why: committing the regenerated docs/story-timeline.md bumps today's commit count → --check was always STALE same-day (committed 5 vs live 6+).
Verified:
- before edit: tools/timeline.py --check → STALE ['story-timeline.md'] exit 1
- after: --check → fresh (81 rows) exit 0 (committed row 2026-10-01 shows 5)
- negative: edited 2026-09-30 Commits 93→94 in docs/story-timeline.md → STALE exit 1; git checkout docs/story-timeline.md → fresh exit 0
Note: mask keys on the latest date; if committed file's last row is an older day, rows differ anyway → STALE (correct).