CI: benchmark regression detection on PR

[benaadams]

Problem

Performance regressions can slip in undetected without automated comparison.

Sub-tasks

• GitHub Actions workflow running BenchmarkDotNet on PRs
• Compare against baseline stored in repo (or previous run)
• Post results as PR comment (e.g., via benchmark-action/github-action-benchmark)
• Warning threshold (not hard gate) for regressions > 5%

Dependencies

Depends on: #134 (full benchmark suite)
Unlocks: performance regression protection

Phase

Phase 4 (Polish & Release Readiness)