Keep going past unreadable files with check --continue (closes #11)

With --continue, check stopped at the first file whose content or
checksum attribute it could not read, and at the first directory it
could not list. Each of these now counts as failed, its error, which
names the path, goes to stderr, and the walk goes on; the run still
exits non-zero. The count that sizes the progress bar leaves such a
path out, so the bar stays. The walk and the count, shared with sum
and clear, take the continue setting; those commands pass false and
still stop at the first error. Without --continue the first such error
still stops the run, and the summary printed before it now counts an
unreadable checksum attribute as failed, as it already did for
unreadable content.

Model: opus-5-5

TODO.md

@@ -24,6 +24,9 @@ today) with lint, fmt, fmt-check, check, and hooks targets.
 
 # Completed Steps
 
+* 2026-10-06: `check --continue` keeps going past a file or directory
+    it cannot read: it counts it as failed, prints the error and the
+    path on stderr, and checks the rest of the tree
 * 2026-10-05: golangci-lint settings take effect: canonical
     `.golangci.yml` (v2 layout, settings under `linters.settings`),
     golangci-lint pinned at v2.12.2 in `Dockerfile` and