test: commit a verify-build failure-mode battery and run it from make check (closes #227)

README.md

@@ -88,13 +88,21 @@ provide:
 - `script/lint` — run the linter
 - `script/fmt` — format all files (writes)
 - `script/fmt-check` — check formatting (read-only)
-- `script/check` — run test, lint, and fmt-check
+- `script/check` — run test, test-verify-build, lint, and fmt-check
 - `script/verify-build` — assert the compiled `DEBUG` state of the bundles in
   `dist/`: every bundle containing `src/shared/constants.js` must have `DEBUG`
   off, or on when `AUTISTMASK_DEBUG=1`. Run automatically at the end of
   `make build` and `make build-debug`; fails loudly rather than passing if it
   cannot determine a bundle's state. Not part of `make check`, which does not
   depend on build artifacts existing.
+- `script/test-verify-build` — exercise every failure mode of
+  `script/verify-build` against a fixture tree in a temp dir, asserting the exit
+  status and the message of each. Part of `make check`; it reads no build
+  artifacts and writes nothing under `dist/`. The cases that depend on file
+  permissions cannot mean anything for a process that is not subject to them, so
+  the harness proves its runner against a mode-000 file before counting them,
+  dropping to an unprivileged user when run as root; if it cannot, it skips
+  those cases and says so in a banner rather than passing them.
 - `script/docker` — build the Docker image tagged via `script/projectname`
 - `script/cibuild` — CI entrypoint: plain `docker build .`
 - `script/precommit` — run by the git pre-commit hook; runs `script/check`