fix(beat-audio): harden voice-provider validation and resolveVoice fast path

Address PR-agent review findings:

- resolveVoice fast path: replace ambiguous boolean comparison
  (voiceProvider === "stepfun") === serverStepfun with explicit
  per-provider equality checks. Prevents an undefined or unknown
  provider from matching the non-stepfun (xiaomi) branch by accident.

- /api/beat-audio route: reject requests whose voice.provider is present
  but not in the VALID_TTS_PROVIDERS whitelist (e.g. "azure"). Previously
  such a request would pass validation when fallback fields were also
  present, and resolveVoice might use the invalid voice directly instead
  of falling back to reprovision — producing a silent beat instead of a
  voiced one.

lib/engine/orchestrator.ts

@@ -258,10 +258,13 @@ async function resolveVoice(
 ): Promise<CharacterVoice | undefined> {
   const serverStepfun = !!config.tts && isStepfun(config.tts);
   const voiceProvider = req.voice?.provider;
+  const voiceMatchesServer =
+    (voiceProvider === "stepfun" && serverStepfun) ||
+    (voiceProvider === "xiaomi" && !serverStepfun);
 
   // Fast path: the client sent a matching voice. (Also covers the legacy
   // xiaomi card + xiaomi server case where the 220KB was unavoidable anyway.)
-  if (req.voice && (voiceProvider === "stepfun") === serverStepfun) {
+  if (req.voice && voiceMatchesServer) {
     return req.voice;
   }