build: assert DEBUG is off in every emitted bundle as a post-build check (closes #170)

build.js records which emitted bundles contain src/shared/constants.js, and
constants.js carries a marker constant-folded from DEBUG itself. script/verify-build
cross-checks the two and fails on every way of not knowing, so deleting the
__BUILD_DEBUG__ define now breaks the build instead of shipping a live debug branch.

README.md

@@ -59,6 +59,13 @@ behavior. The build prints which mode it used. See the
 distribute a debug build** — every wallet it creates gets the same publicly
 known test recovery phrase.
 
+Both builds end by running `script/verify-build`, which reads the compiled
+`DEBUG` state back out of the emitted bundles and fails the build if it is not
+the one that was asked for. The test suite cannot check this: it loads
+`src/shared/constants.js` outside a bundle, so it only ever sees the fallback
+value. The assertion is on the artifacts because that is where the property
+lives.
+
 ## Entrypoints
 
 This repository adheres to the
@@ -79,6 +86,12 @@ provide:
 - `script/fmt` — format all files (writes)
 - `script/fmt-check` — check formatting (read-only)
 - `script/check` — run test, 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/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`