security: make DEBUG a build-time flag defaulting to off (closes #149)

DEBUG was hardcoded to true in src/shared/constants.js, so every wallet
created from a build of main received the publicly committed test recovery
phrase and was instantly drainable. There was no way to produce a non-debug
build at all: the real entropy path in generateMnemonic() was dead code in
every artifact.

DEBUG is now a build-time constant injected by esbuild's define in build.js,
alongside the existing __BUILD_* defines, and read by constants.js with the
same typeof guard buildInfo.js uses. It is false unless the build was run
with AUTISTMASK_DEBUG=1 — an exact match, so an unset, empty or mistyped
value fails safe towards a release build. The build prints which mode it
used, and make build-debug is a shim for the debug case.

What DEBUG does when enabled is unchanged: the red banner plus the hardcoded
test phrase, no new conditionals. Mnemonic generation deliberately keeps
reading the compile-time constant rather than isDebug() from log.js, which
also ORs in the runtime debugMode flag the settings toggle drives; routing it
through isDebug() would let a user of a release build re-enable the known
test phrase for real wallets. That is now recorded at the call site, in the
README DEBUG Mode Policy, and covered by a regression test.

New tests/wallet.test.js covers both build modes: with the flag off, two
successive generateMnemonic() calls differ, both validate as BIP-39 phrases,
both are 12 words, neither is DEBUG_MNEMONIC, and the result still derives a
usable HD wallet — including with the runtime toggle forced on. With the flag
on, DEBUG is true and the test phrase is returned, so the debug path stays
proven rather than silently removed.

Verified with make check (55 tests, lint, fmt-check all green), and with
make build and make build-debug: all four bundles across dist/chrome and
dist/firefox export DEBUG:!1 in a release build and DEBUG:!0 in a debug
build, and AUTISTMASK_DEBUG=true likewise yields DEBUG:!1.

build.js

@@ -11,6 +11,14 @@ function ensureDir(dir) {
     fs.mkdirSync(dir, { recursive: true });
 }
 
+// DEBUG is a build-time flag, off unless explicitly requested. It is the only
+// thing that makes the hardcoded test mnemonic reachable, so the opt-in must be
+// exact: anything other than the literal "1" (unset, empty, "true", a typo)
+// produces a release build. Failing towards the safe mode is deliberate.
+function isDebugBuild() {
+    return process.env.AUTISTMASK_DEBUG === "1";
+}
+
 function getBuildInfo() {
     const pkg = JSON.parse(
         fs.readFileSync(path.join(__dirname, "package.json"), "utf8"),
@@ -47,7 +55,15 @@ async function build() {
     const buildInfo = getBuildInfo();
     console.log("Build info:", buildInfo);
 
+    const debugBuild = isDebugBuild();
+    console.log(
+        debugBuild
+            ? "Build mode: DEBUG (INSECURE - hardcoded test mnemonic, do not ship)"
+            : "Build mode: release (DEBUG off)",
+    );
+
     const define = {
+        __BUILD_DEBUG__: JSON.stringify(debugBuild),
         __BUILD_VERSION__: JSON.stringify(buildInfo.version),
         __BUILD_LICENSE__: JSON.stringify(buildInfo.license),
         __BUILD_AUTHOR__: JSON.stringify(buildInfo.author),