test(10): complete UAT evidence

[tazarov]

Summary

• mark phase 10 UAT complete with 8/8 checks passing
• record live GitHub Actions evidence for baseline, PR benchmark, cache-miss, and concurrency behavior
• document restored baseline cache after the cache-miss verification

Verification

• python3 tests/bench/test_pr_benchmark_workflows.py
• git diff --check
• live Actions runs recorded in .planning/phases/10-lightweight-pr-benchmark-regression-signal/10-UAT.md

Merge

Use squash merge.