feat(voice-call): pre-cache inbound greeting for instant playback

Pre-generates TTS audio for the configured inboundGreeting at startup
and serves it instantly when an inbound call connects, eliminating the
500ms+ TTS synthesis delay on the first ring.

Changes:
- twilio.ts: Add cachedGreetingAudio storage with getter/setter
- runtime.ts: Pre-synthesize greeting TTS after provider initialization
- webhook.ts: Play cached audio directly via media stream on inbound
  connect, falling back to the original TTS path for outbound calls
  or when no cached audio is available

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

extensions/voice-call/src/providers/twilio.ts

@@ -62,6 +62,17 @@ export class TwilioProvider implements VoiceCallProvider {
 
   /** Map of call SID to stream SID for media streams */
   private callStreamMap = new Map<string, string>();
+  /** Pre-generated greeting audio for instant inbound playback */
+  private cachedGreetingAudio: Buffer | null = null;
+
+  setCachedGreetingAudio(audio: Buffer): void {
+    this.cachedGreetingAudio = audio;
+    console.log(`[voice-call] Cached greeting audio: ${audio.length} bytes`);
+  }
+
+  getCachedGreetingAudio(): Buffer | null {
+    return this.cachedGreetingAudio;
+  }
   /** Per-call tokens for media stream authentication */
   private streamAuthTokens = new Map<string, string>();