docs(statusline): record dirty-check timeout evidence

[SUaDtL]

Summary

• Benchmark the shipped statusline dirty probe across five repositories and four focused primary or linked-worktree cases.
• Retain the 100 ms full-porcelain timeout after 600 measured probes produced zero timeouts and zero dirty-state mismatches.
• Record the exact reproduction contract and anonymous per-case receipts, then complete the tracked benchmark task.

This PR is stacked on #362 so the evidence rides the current ca-pi release chain without duplicating its inherited changes.

Test plan

• Run all 42 .github/scripts/test_*.py suites: pass.
• Run python -m unittest discover -s plugins/ca/hooks/tests -p test_*.py: 1,023 pass.
• Repeat the 600-probe production-budget check: zero timeouts and zero mismatches.
• Run the required coverage audit: pass after adding reproducible per-case receipts.
• Run staged diff, secret-pattern, identity-confinement, and board-transition checks: pass.

Tradeoff

Conflict hierarchy level 2, implementation policy: raising the timeout could reduce fail-soft risk but would expand prompt-refresh latency. SMARTS favors retaining the bounded 100 ms policy because the largest available checkout stayed below the deadline with zero hidden dirty states, while the report limits the conclusion to the measured repository range.

[SUaDtL]

Superseded by #313, the single hackathon review surface. This PR's recorded head and changes are represented in #313's exact 16-PR / 29-commit / 126-path source manifest. PR #313 passed final exact-head CI at commit 6173b1d and remains open and unmerged. Closing this source PR to avoid parallel review; please continue review and discussion on #313.