fix: NUL-delimit verify-build's dist walk so no path escapes the check (closes #223)

check_unlisted_bundles read dist/ line by line, so two unlisted paths
carrying a debug marker went unchecked while the script still exited 0:
a name with a trailing space (read stripped it and the remnant matched a
manifest line) and a name containing a newline (find printed it as a
listed path plus an empty one).

The walk is now find -print0 into a temporary listing, checked for find's
exit status as before, and xargs -0 hands the paths back to this script as
arguments, where the same is_listed and has_marker run on them. is_listed
also answers "not listed" for any path containing a newline without asking
grep, which would otherwise read such a path as two patterns and match on
the first half — the escape survives NUL delimiting on its own.

dist/ being a symlink is now its own check with its own message. It failed
before only because GNU grep exits 2 on a directory, so a grep that exits 1
instead would have turned the whole cross-check into a pass.

The comment claiming every file under dist/ is searched now says what is
actually guaranteed: every regular file and every symlink, with the four
properties that coverage rests on stated as enforced rather than assumed.

TODO.md

@@ -44,6 +44,10 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-11: `script/verify-build` now walks `dist/` NUL-delimited and asserts
+  `dist/` is a real directory, so a path with a trailing space or a newline can
+  no longer carry a debug marker past the unlisted-bundle check
+  ([#223](https://git.eeqj.de/sneak/AutistMask/issues/223)).
 - 2026-08-11: A dust threshold of `0` now means "hide nothing" instead of
   falling back to the 100,000 gwei default, and every address comparison in
   `src/shared/transactions.js` goes through one case-normalising helper so a