fix: populate ctime from actual filesystem stats instead of mtime fallback

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

internal/snapshot/ctime_darwin.go

@@ -0,0 +1,26 @@
+package snapshot
+
+import (
+	"os"
+	"syscall"
+	"time"
+)
+
+// fileCTime returns the file creation time (birth time) on macOS.
+//
+// On macOS/Darwin, "ctime" refers to the file's birth time (when the file
+// was first created on disk). This is stored in the Birthtimespec field of
+// the syscall.Stat_t structure.
+//
+// This differs from Linux where "ctime" means inode change time (the last
+// time file metadata was modified). See ctime_linux.go for details.
+//
+// If the underlying stat information is unavailable (e.g. when using a
+// virtual filesystem like afero.MemMapFs), this falls back to mtime.
+func fileCTime(info os.FileInfo) time.Time {
+	stat, ok := info.Sys().(*syscall.Stat_t)
+	if !ok {
+		return info.ModTime()
+	}
+	return time.Unix(stat.Birthtimespec.Sec, stat.Birthtimespec.Nsec).UTC()
+}