[TASK] FastWalker benchmark (AC-30)

[kirich1409]

Context

Benchmark FastWalker on synthetic fixture of 100k files across 1000 directories on Apple Silicon SSD. Target <5s.

Acceptance criteria

• AC-30 passes on CI runner.
• Results exported as JSON artifact.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

• Blocked by [TASK] Parallel walk across projectRoots (bounded 4-8) #59
• Blocked by [SPIKE] benchmark framework choice #88

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved