Consolidate filesystem walkers across platforms

[m3nu]

Context

We currently maintain two filesystem walker implementations for backup:

1. Inode-sorted walker (inode_walk.rs) — Linux only (#[cfg(target_os = "linux")]). Sorts stat() calls by inode number for 3-8x HDD speedup on ext4/xfs/reiserfs. Manages its own gitignore stack, exclude patterns, and marker file filtering.

2. ignore-crate walker (walk/mod.rs, build_configured_walker) — non-Linux fallback. Delegates gitignore filtering to the ignore crate's WalkBuilder, with our own filter_entry closure for exclude patterns, marker files, and cross-device filtering.

This dual implementation causes a practical gap: on non-Linux, gitignore-excluded files can't be logged at debug level because the ignore crate filters them internally before our code sees them (discovered in #73).

Proposal

Consolidate to a single walker implementation across all platforms:

• Extend the inode walker to all Unix — change cfg(target_os = "linux") to cfg(unix). Make inode_sort_beneficial() return false on non-Linux (no ext4/xfs detection needed). macOS gets filename-order DFS with full gitignore logging.

• Extend to Windows — the core walk logic (read_dir, symlink_metadata, DFS stack) is cross-platform. Windows-specific: use dummy inode (0, no sorting), skip statfs, handle one_file_system/device via platform abstraction.

• Keep ignore::gitignore — we still need the ignore crate for gitignore pattern matching (Gitignore, GitignoreBuilder). Only the WalkBuilder walker would be dropped.

• Remove build_configured_walker and walk_source_ignore — replaced by the consolidated walker. The existing filter_equivalence_with_ignore_walker test can be kept as a regression test or adapted.

Benefits

• Single code path for filtering logic (excludes, gitignore, markers, cross-device)
• Consistent debug logging of excluded files on all platforms
• Less code to maintain

Considerations

• Needs Windows CI testing
• one_file_system semantics may differ on Windows (drive letters vs device IDs)
• The ignore crate dependency remains for pattern matching

Related: #73