fix: preserve macOS bulk metadata fallback and stat semantics

[huntharo]

Main already uses getattrlistbulk for macOS enumeration. This change adds fallback and metadata compatibility handling to that path.

When the first bulk call returns ENOTSUP, ENOSYS or EACCES, the reader falls back to std::fs::read_dir, preserving name enumeration and per-entry metadata errors. It never restarts after returning entries, which would duplicate part of the tree.

Additional behavior:

• Pre-epoch modification times map to zero, matching the standard scanner’s timestamp handling.
• Firmlink records and missing attributes use visible-path, no-follow metadata; directory checks run before descent.
• Packed records validate returned attribute masks, field bounds and filename boundaries, retaining raw OS error codes.

Tests cover malformed and truncated records, omitted attributes, fallback before/after partial enumeration, permissions, timestamps, firmlink fixtures, disappearing directories, multiple buffers and accounting parity.

Validation at b36bad6: 157 core tests passed (2 ignored), 71 app tests passed, and cargo +1.97.1 xtask lint passed.

Tradeoff and limits: the current patch replaces the macOS reader, adds a direct macOS libc dependency and does not retain main’s per-thread buffer reuse. No performance improvement over current main has been established. Unsupported-enumeration fallback and firmlink cases include injected fixtures; a real unsupported filesystem and live evicted cloud-provider behavior remain untested.