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

TODO.md

@@ -45,6 +45,16 @@ but the review is broader than any of them.
 
 # Completed Steps
 
+- 2026-09-21: The test recovery phrase no longer survives in a release bundle,
+  and the committed-key guard matches by content
+  ([#351](https://git.eeqj.de/sneak/AutistMask/issues/351)). `DEBUG_MNEMONIC` in
+  `src/shared/constants.js` is now behind the `__BUILD_DEBUG__` define, so a
+  release build folds the phrase to `null` and no emitted bundle carries it; it
+  used to survive as dead text because `module.exports` keeps the const alive.
+  `script/verify-build` now fails a release build if the phrase appears in any
+  emitted file, so the fold cannot silently regress. `tests/extensionId.test.js`
+  scans the content of every tracked file for a PEM private-key header instead
+  of matching filename extensions alone.
 - 2026-09-21: A transaction response is honoured only for a transaction
   approval, and the three remaining approval-settlement paths are pinned
   ([#262](https://git.eeqj.de/sneak/AutistMask/issues/262)). The liveness fix
@@ -58,7 +68,6 @@ but the review is broader than any of them.
   approve, reject and disconnect paths against a transaction approval
   broadcasting behind them: each is declined and the dApp still receives its
   broadcast result.
-
 - 2026-08-30: An address no longer wraps, or is shortened to fit, in any of the
   common views ([#380](https://git.eeqj.de/sneak/AutistMask/issues/380)). The
   wallet list was the reported case: the address shared one row with the