fix(tts): make stepfun preset pick case-stable and per-character

- Hash the lowercased description (matching the case-insensitive scoring)
  so the same archetype text picks the same preset regardless of case.
- Thread the character name through provisionVoice -> stepfunProvision as
  the hash salt, so two characters that share archetype keywords spread
  across the top-N candidate presets instead of collapsing on one voice.

Xiaomi path is unaffected (voicedesign mints a unique clip per call).

lib/engine/agents/characterDesigner.ts

@@ -103,7 +103,7 @@ export async function provisionCharacterVoice(
 ): Promise<CharacterVoice | undefined> {
   if (!config.tts) return undefined;
   try {
-    return await provisionVoice(config.tts, voiceDescription);
+    return await provisionVoice(config.tts, voiceDescription, charName);
   } catch (err) {
     const msg = err instanceof Error ? err.message : String(err);
     console.error(`[characterDesigner] voice provision failed for ${charName}: ${msg}`);