[TASK] FastWalkerImpl via getattrlistbulk (P1)

[kirich1409]

Context

DevsweepCorePrivate.FastWalkerImpl uses open(O_DIRECTORY) + getattrlistbulk(2) batch syscalls. Target: 3-7× faster than FileManager.enumerator.

Acceptance criteria

• Implementation uses getattrlistbulk.
• Correctness tests pass (enumeration matches FileManager reference).
• Benchmark: <5s on 100k files on Apple Silicon SSD (precondition for AC-30).

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

• Blocked by [TASK] FastWalker protocol + default exclusion constants #57

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