Stop restore --verify re-reporting files that failed to restore (closes #323)

With --skip-errors, a file that failed to restore was verified anyway,
failed again because it was never written, and was added to the same
failure list as the restore failure. restore --verify then listed it
twice and called every entry a checksum mismatch.

Verification now skips the files that failed to restore and keeps its
own failures in a separate list, so failed restores and failed
verifications are reported apart, each path once. The restore still
exits non-zero. The tests cover a restore with skipped files where
nothing fails verification, and one where another restored file also
fails it.

Judgement call: the second test has its own trailing-byte file-system
wrapper; the one in restore_perms_test.go is in the internal test package.

Model: opus-5-5

README.md

@@ -375,7 +375,9 @@ on the destination in one go, use `vaultik remote nuke --force`.
 * Optional path arguments to restore specific files/directories (default: all)
 * Preserves file permissions, timestamps, ownership (ownership requires root),
   symlinks, and empty directories
-* `--verify`: After restoring, verify every file's chunk hashes match
+* `--verify`: After restoring, verify every restored file's chunk hashes
+  match. A file skipped under `--skip-errors` is reported as failed to
+  restore and is not verified.
 
 **`prune`**: Tidy up everything that isn't needed. Runs three passes:
 (1) reconcile the local index against the destination store — any