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

src/shared/constants.js

@@ -22,8 +22,15 @@ const BUILD_DEBUG_MARKER = DEBUG
     ? "autistmask-build-debug=on"
     : "autistmask-build-debug=off";
 
-const DEBUG_MNEMONIC =
-    "cube evolve unfold result inch risk jealous skill hotel bulb night wreck";
+// Behind DEBUG for the same reason BUILD_DEBUG_MARKER is above: in a release
+// build __BUILD_DEBUG__ is a compile-time false, esbuild drops this branch, and
+// the phrase never reaches a distributed bundle. The literal used to survive as
+// dead text because module.exports keeps this const live even though wallet.js's
+// only use of it is folded away; making the value itself fold to null removes
+// it. script/verify-build fails a release build if the phrase appears anyway.
+const DEBUG_MNEMONIC = DEBUG
+    ? "cube evolve unfold result inch risk jealous skill hotel bulb night wreck"
+    : null;
 
 const ETHEREUM_MAINNET_CHAIN_ID = "0x1";
 const ETHEREUM_SEPOLIA_CHAIN_ID = "0xaa36a7";