feat(voice): on-device voice tier — Gemma 4, Moonshine and Kokoro in the browser

src/voice/local/localWebProvider.js

@@ -0,0 +1,71 @@
+export { LOCAL_WEB_VOICE, localWebVoiceRequested } from './localWebFlag.js';
+
+const cancelled = () =>
+  new DOMException('On-device voice was stopped', 'AbortError');
+
+/**
+ * Session adapter factory for the on-device tier that loads the provider on
+ * first use, so pages without the flag never download it. A stop issued
+ * while the module loads cancels the pending start; after removal the
+ * provider is never constructed and every later start is refused.
+ */
+export function createLocalWebSessionLoader(
+  hooks,
+  {
+    load = () =>
+      Promise.all([
+        import('./localWebSession.js'),
+        import('../../ui/styles/local-voice.css'),
+      ]).then(([module]) => module),
+  } = {},
+) {
+  let adapter = null;
+  let bound = false;
+  let disposed = false;
+  let stopEpoch = 0;
+  const isDisposed = () => disposed || Boolean(hooks.signal?.aborted);
+  const ready = Promise.resolve()
+    .then(load)
+    .then((module) => {
+      if (isDisposed()) return null;
+      adapter = module.createLocalWebSession(hooks);
+      if (bound) adapter.bindControls();
+      return adapter;
+    });
+  ready.catch(() => {});
+  return {
+    capabilities: { costControls: false, pushToTalk: true, local: true },
+    get local() {
+      return adapter?.local || null;
+    },
+    ready,
+    async start(options) {
+      if (isDisposed()) throw cancelled();
+      const epoch = stopEpoch;
+      const loaded = await ready;
+      if (!loaded || isDisposed()) throw cancelled();
+      if (epoch !== stopEpoch) return undefined;
+      return loaded.start(options);
+    },
+    stop(options = {}) {
+      stopEpoch++;
+      if (options.removeUi) disposed = true;
+      adapter?.stop(options);
+    },
+    sendText(text) {
+      if (isDisposed()) return false;
+      void ready.then((loaded) => {
+        if (loaded && !isDisposed()) loaded.sendText(text);
+      });
+      return true;
+    },
+    sendMapEvent() {
+      return false;
+    },
+    ignoreButtonClick: () => Boolean(adapter?.ignoreButtonClick()),
+    bindControls() {
+      bound = true;
+      if (!isDisposed()) adapter?.bindControls();
+    },
+  };
+}