fix(test-vendor): append the zerolag global aliases

The zerolag bundle exports only `XtermZerolagInput`, but app.js constructs
`new LocalEchoOverlay(terminal)` directly. scripts/build.mjs appends global
aliases after esbuild (build.mjs:53-66); the first version of this script
omitted that step.

Without them initTerminal() throws `LocalEchoOverlay is not defined` at the
line that builds the overlay — and because that is midway through the function,
EVERY later step silently never runs, including the mobile touch handlers on
#terminalContainer. The page still had a terminal, so the failure looked like a
tap-routing bug rather than a boot error.

Verified: boot errors none, and all four terminalContainer touch listeners
(touchstart/touchmove/touchend/touchcancel) now register.

scripts/prepare-test-vendor.mjs

@@ -19,7 +19,7 @@
  * Idempotent: skips outputs already newer than their source.
  */
 import { execFileSync } from 'node:child_process';
-import { copyFileSync, existsSync, mkdirSync, statSync } from 'node:fs';
+import { appendFileSync, copyFileSync, existsSync, mkdirSync, statSync } from 'node:fs';
 import { dirname, join, resolve } from 'node:path';
 import { fileURLToPath } from 'node:url';
 
@@ -100,6 +100,27 @@ for (const asset of ASSETS) {
     );
   }
   built += 1;
+
+  // The zerolag bundle exports only `XtermZerolagInput`. app.js constructs
+  // `new LocalEchoOverlay(terminal)` directly, so scripts/build.mjs appends
+  // global aliases after esbuild — without them initTerminal() throws
+  // `LocalEchoOverlay is not defined` at the point it builds the overlay, and
+  // every later step (including the mobile touch handlers) silently never runs.
+  if (asset.out === 'xterm-zerolag-input.js') {
+    appendFileSync(
+      dest,
+      '\n// Global aliases for browser usage\n' +
+        'if(typeof window!=="undefined"){' +
+        'window.ZerolagInputAddon=XtermZerolagInput.ZerolagInputAddon;' +
+        'window.LocalEchoOverlay=class extends XtermZerolagInput.ZerolagInputAddon{' +
+        'constructor(terminal){' +
+        'super({prompt:{type:"character",char:"\\u276f",offset:2}});' +
+        'this.activate(terminal);' +
+        '}' +
+        '};' +
+        '}\n'
+    );
+  }
 }
 
 console.log(`[test-vendor] ${built} built, ${skipped} up to date -> src/web/public/vendor/`);