Store mtime in nanoseconds so a same-second rewrite is re-hashed (closes #12)

scan recorded mtime in whole seconds, so a file rewritten in place at
the same size within the same second as its recorded mtime was classed
unchanged and kept its old hashes. The walk, a file given as an operand,
and the content phase's recheck now all use ModTime().UnixNano(); the
change rule itself is unchanged. The column stays INTEGER and
PRAGMA user_version stays 1, per the owner's ruling. A database written
by an earlier build holds seconds, so its files count as changed once,
at the next scan that covers them. README states the new resolution.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- `scan` records mtime in Unix nanoseconds and compares it at that resolution,
+  so a same-size rewrite within the same second is re-hashed (2026-10-07,
+  https://git.eeqj.de/sneak/sfdupes/issues/12)
+
 - cut the narration from `TODO.md` Completed Steps and from the comments in
   `script/` and both Dockerfiles; §Workflow now branches from and merges to
   `next` (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/49)