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

tests/wallet.test.js

@@ -27,6 +27,14 @@ describe("generateMnemonic in a release build", () => {
         expect(constants.DEBUG).toBe(false);
     });
 
+    test("the test phrase folds away when DEBUG is false", () => {
+        // The release bundle is what must not carry the phrase; here, with the
+        // define absent, DEBUG_MNEMONIC is the null branch the bundler keeps,
+        // and the literal only exists in the branch it drops.
+        const { constants } = loadWallet();
+        expect(constants.DEBUG_MNEMONIC).toBeNull();
+    });
+
     test("returns fresh, valid 12-word phrases that are not the test phrase", () => {
         const { constants, wallet } = loadWallet();