Add path expansion module (expand.rs) with:
- FileWorkItem and PathOrUrl types for work items
- expand_paths() function for directory traversal via walkdir
- Case-insensitive *.pdf filtering
- Hidden directory skip (. prefix)
- Remote URL support when feature enabled
- bytes_total calculation for progress reporting
Fix event.rs should_skip_confidence() for proper NaN handling.
All 130 grep tests pass. See notes/pdftract-3gf5t.md for details.
