harden: keep the test recovery phrase out of release bundles, match committed keys by content (closes #351)

The 12-word BIP-39 test phrase survived in every release bundle as dead
text: module.exports keeps DEBUG_MNEMONIC live even though wallet.js's only
use of it folds away in a release build, so it could not be tree-shaken.
Putting the value itself behind the __BUILD_DEBUG__ define makes esbuild fold
it to null, so no distributed bundle carries it. script/verify-build now
fails a release build if the phrase appears in any emitted file, so the fold
cannot silently regress; test-verify-build covers both the release failure
and the debug allowance.

tests/extensionId.test.js now scans the content of every tracked file for a
PEM private-key header instead of matching filename extensions alone, so a
key committed under an unexpected name is caught.

Model: opus-4-8

script/test-verify-build

@@ -37,6 +37,10 @@ DISCARD_DIST="$ROOT/script/discard-dist-on-failure"
 MARKER_ON="autistmask-build-debug=on"
 MARKER_OFF="autistmask-build-debug=off"
 
+# The same test recovery phrase verify-build searches release bundles for. Held
+# here too, the way the markers above are, so a case can plant it in a bundle.
+TEST_MNEMONIC="cube evolve unfold result inch risk jealous skill hotel bulb night wreck"
+
 RECEIPT_HEADER="autistmask-build-receipt v1"
 
 NEWLINE='
@@ -516,6 +520,24 @@ c_debug_build() {
     write_receipt
 }
 
+# A release bundle that still carries the test recovery phrase — the regression
+# verify-build guards against, and the reason DEBUG_MNEMONIC is behind the
+# __BUILD_DEBUG__ define in src/shared/constants.js. The receipt is regenerated
+# so the phrase is caught as bundle content, not incidentally as a stale digest.
+c_release_bundle_with_mnemonic() {
+    printf '/* %s */\n' "$TEST_MNEMONIC" >>dist/chrome/src/popup/index.js
+    write_receipt
+}
+
+# The same phrase in a debug build is expected: make build-debug ships it on
+# purpose, so the phrase check must stay quiet under --expect debug.
+c_debug_bundle_with_mnemonic() {
+    write_bundle dist/chrome/src/popup/index.js "$MARKER_ON"
+    write_bundle dist/firefox/src/popup/index.js "$MARKER_ON"
+    printf '/* %s */\n' "$TEST_MNEMONIC" >>dist/chrome/src/popup/index.js
+    write_receipt
+}
+
 c_no_dist() { rm -rf dist; }
 
 # --- dist discard -----------------------------------------------------------
@@ -753,6 +775,13 @@ run_cases() {
     check_case "debug bundles under --expect debug pass" \
         no debug 0 "2 bundle(s) $MARKER_ON" c_debug_build
 
+    check_case "release bundle carrying the test recovery phrase fails" \
+        no release 1 \
+        "carries the BIP-39 test recovery phrase" c_release_bundle_with_mnemonic
+
+    check_case "debug bundle carrying the test recovery phrase passes" \
+        no debug 0 "2 bundle(s) $MARKER_ON" c_debug_bundle_with_mnemonic
+
     check_case "no --expect argument" \
         no no-expect 1 "no --expect argument." c_control