test: cover every verify-build failure mode from make check (closes #227)

script/verify-build is the build-integrity guard and nothing in make check
tested it. Three separate reviews of it each found a fresh vacuous pass --
the grep exit-2 conflation, the discarded find status, the line-delimited
walk -- and each was caught by someone building a tree by hand.

script/test-verify-build makes that battery a target. It builds a fixture
tree under mktemp -d holding script/verify-build as a symlink to the real
script: verify-build takes its ROOT from dirname "$0"/.., so it runs against
the fixture's dist/ and never touches the repo's build output, and the
symlink rather than a copy is what makes a break in the real script fail
here. Its own mktemp is pointed inside the work dir, so a run leaves nothing
behind. Each of the 18 cases asserts the exit status AND a substring of the
message, because a guard that fails for the wrong reason is itself a defect.

The obstacle was that the CI image declares no USER, so CI runs as root, and
root is not subject to file permissions: a chmod 000 case passes vacuously
there while passing honestly on a developer machine. The harness therefore
proves its runner before counting the five permission-dependent cases -- a
mode-644 file and script/verify-build itself must be readable through it, a
mode-000 file must not be -- and drops to uid 65534 via setpriv or runuser
when running as root, both present in the pinned base image. If no candidate
passes the probes the cases are skipped, named in a banner, and the final
line says SKIPPED AND NOT PROVEN rather than passed. A green run cannot mean
"the permission cases did not run" without saying so.

TODO.md

@@ -44,6 +44,12 @@ undefined identifiers, which is how
 
 # Completed Steps
 
+- 2026-08-12: `script/verify-build`'s failure modes are now a committed target,
+  `script/test-verify-build`, run by `make check`. It asserts the exit status
+  and the message of every case against a fixture tree in a temp dir, and drops
+  privileges (proving the runner against a mode-000 file first) for the cases
+  that only mean something when file permissions are in force
+  ([#227](https://git.eeqj.de/sneak/AutistMask/issues/227)).
 - 2026-08-12: Bundled token list documentation no longer states a count. The
   four "top 250" claims in `README.md` and the "roughly 500" claim in
   `docs/README.md` are replaced with a description of how the list is actually