Hash a file again when its mtime moved back, not only forward (closes #176)

scan treated a recorded file as unchanged when its size matched and its
mtime was not newer. A file replaced by other content of the same size
with an older mtime, as cp -p, rsync -a or a restore writes it, kept the
hash of the file it replaced. Any mtime difference now counts as a
change. README.md says so, and a new test covers the older mtime.

Model: opus-5-5

TODO.md

@@ -29,6 +29,11 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- `scan` hashes a file again when its mtime differs from its record, older or
+  newer. Before, an older mtime counted as unchanged, so a file put back by
+  `cp -p` or a restore kept the hash of the file it replaced (2026-11-11,
+  https://git.eeqj.de/sneak/sfdupes/issues/176)
+
 - The Go code moved out of the repository root. `cmd/sfdupes/main.go` is the
   command, and it calls `Main` in the package `internal/sfdupes`. `main.go`
   became `internal/sfdupes/cli.go`. Every other file kept its name. The install