Replace the mtime fallback for ctime in the scanner with platform-specific
implementations that extract the real ctime from syscall.Stat_t:
- macOS/Darwin: uses Birthtimespec (file birth/creation time)
- Linux: uses Ctim (inode change time)
- Falls back to mtime when syscall stats are unavailable (e.g. afero.MemMapFs)
Also:
- Document platform-specific ctime semantics in README
- Document ctime restore limitations (cannot be set on either platform)
- Add ctime field documentation to File model
- Update README files table schema to match actual schema (adds ctime,
source_path, link_target columns)
- Add comprehensive tests for fileCTime on real files and mock FileInfo
closes#13
