[TASK] Parallel walk across projectRoots (bounded 4-8)

[kirich1409]

Context

withTaskGroup with bounded concurrency (4-8) across multiple projectRoots. Avoids APFS CoW lock contention from unbounded parallelism.

Acceptance criteria

• Multi-root synthetic benchmark shows sub-linear-degradation scaling.
• Cancellation via Task.checkCancellation propagates.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

• Blocked by [TASK] FastWalkerImpl via getattrlistbulk (P1) #58

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