Propagate overlay directory listing errors

[smigolsmigol]

Overlay iterdir returned an empty list when the host directory listing failed.
Direct mounts return the mapped error, so permission, not-found, not-a-directory, and entry iteration failures could appear as an empty directory only in overlay mode.

Propagate the error returned by host_list_visible_dir_entry_names.
The helper's existing symlink filtering and memory accounting remain unchanged.

The existing direct and overlay permission parity test now covers Iterdir.

Tests

• cargo test -p monty-fs overlay_permission_errors_match_direct_mode
• cargo test -p monty-fs
• make format-rs
• make lint-rs

---

Summary by cubic

Propagates host directory listing errors from overlay iterdir to match direct mounts. Previously overlay returned an empty list on host listing failure; now it returns the mapped error (permission, not-found, not-a-directory, iteration).

• Return the host error from host_list_visible_dir_entry_names; keep symlink filtering and memory accounting unchanged.
• Extend the overlay_permission_errors_match_direct_mode test to cover iterdir.
• Migration: callers that treated an empty list as success must handle iterdir errors in overlay mode.

^{Written for commit ee50785. Summary will update on new commits.}

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks
⏩ 16 skipped benchmarks¹

---

_{Comparing smigolsmigol:fix/overlay-iterdir-errors (ee50785) with main (edeb82a)}

Footnotes

1. 16 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}