build: remove dist/ when a release build fails (closes #333)

With AUTISTMASK_DEBUG=1 exported in the calling shell, make build compiled a
debug bundle and failed on it in script/verify-build, but left the bundle in
dist/: loadable, with every wallet it creates using the publicly committed test
recovery phrase from src/shared/constants.js. A failed release build that leaves
a loadable debug build behind is the trap the verifier exists to close.

Every step of make build now runs through script/discard-dist-on-failure, which
removes dist/ when its step fails and says on stderr that it did and why, then
returns the step's own exit status. A removal it cannot complete is reported as
loudly as one it can, naming what is still on disk. A step that succeeds removes
nothing, including the final check-censored --require-dist pass. It composes
with the existing receipt trap: the receipt is still deleted on the way out.

make build-debug is deliberately not wrapped. A debug build that failed is not
producing an artifact mistakable for a release one, and its dist/ is the
evidence of what went wrong.

script/test-verify-build asserts the state of dist/ on disk after a failing and
a succeeding step rather than the exit status alone, plus a step that fails with
no dist/ and a wrapper handed no command, and reads make -n to check the wrapper
is on the release path and absent from the debug one. Both directions were also
run end to end: AUTISTMASK_DEBUG=1 make build fails and leaves no dist/, plain
make build passes with all 15 emitted files intact, and a debug build failed
mid-write keeps its dist/.

verify-build itself is unchanged; this is only what happens after it says no.

Makefile

@@ -60,19 +60,31 @@ hooks:
 # scrubbed from the build itself: with AUTISTMASK_DEBUG=1 exported, this target
 # compiles a debug bundle and then fails on it, loudly, rather than quietly
 # handing back something other than the release build that was asked for.
+#
+# Every step of this target is wrapped in script/discard-dist-on-failure, so a
+# release build that fails removes dist/ instead of leaving a complete, loadable
+# debug bundle there for whoever runs the build, sees it fail, and loads
+# dist/chrome/ anyway. A step that succeeds removes nothing, and build-debug is
+# deliberately not wrapped.
 build:
 	@echo "Building extension..."
 	@set -eu; \
 	receipt="$$(mktemp "$${TMPDIR:-/tmp}/autistmask-build-receipt.XXXXXX")"; \
 	trap 'rm -f "$$receipt"' EXIT INT TERM; \
-	AUTISTMASK_BUILD_RECEIPT="$$receipt" yarn run build 2>&1; \
-	env -u AUTISTMASK_DEBUG script/verify-build --expect release \
+	script/discard-dist-on-failure \
+		env AUTISTMASK_BUILD_RECEIPT="$$receipt" yarn run build 2>&1; \
+	script/discard-dist-on-failure \
+		env -u AUTISTMASK_DEBUG script/verify-build --expect release \
 		--receipt "$$receipt"
-	@script/check-censored --require-dist
+	@script/discard-dist-on-failure script/check-censored --require-dist
 
 # Development-only build: enables the red DEBUG / INSECURE banner and makes
 # the hardcoded test recovery phrase the output of wallet creation. Never
 # distribute the artifacts this produces.
+#
+# No discard-dist-on-failure here, on purpose: a debug build that fails is not
+# producing an artifact anyone could mistake for a release one, and its dist/ is
+# the evidence of what went wrong.
 build-debug:
 	@echo "Building extension (DEBUG)..."
 	@set -eu; \