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 walk shared with sum and clear takes the continue
setting, and they pass false. If the count that sizes the progress bar
cannot read a path, check --continue runs without the bar. 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: `make try` runs on three small files in a temporary
     directory that it removes afterwards, also when a step fails,
     instead of on a fixed directory on one person's machine