feat(asr): add StepFun ASR SSE transcription provider

- Add StepFunTranscriptionProvider class in nanobot/providers/transcription.py
- New _post_stepfun_asr_with_retry() function handling SSE stream parsing
  (transcript.text.delta → transcript.text.done event sequence)
- Register 'stepfun' in transcription_registry.py with default model stepaudio-2.5-asr
- Reuse existing stepfun provider config (apiBase can point to Plan endpoint)
- Add 17 tests covering SSE parsing, retry contract, empty-text edge case, and registry integration
- Update docs/configuration.md with stepfun ASR documentation

StepFun ASR uses a dedicated SSE endpoint (/v1/audio/asr/sse) rather
than the chat-completions or Whisper multipart formats used by other
providers. Users on Step Plan can set apiBase to the Plan endpoint.

nanobot/audio/transcription_registry.py

@@ -64,6 +64,11 @@ TRANSCRIPTION_PROVIDERS: tuple[TranscriptionProviderSpec, ...] = (
         adapter="nanobot.providers.transcription:XiaomiMiMoTranscriptionProvider",
         aliases=("mimo", "xiaomi"),
     ),
+    TranscriptionProviderSpec(
+        name="stepfun",
+        default_model="stepaudio-2.5-asr",
+        adapter="nanobot.providers.transcription:StepFunTranscriptionProvider",
+    ),
     TranscriptionProviderSpec(
         name="assemblyai",
         default_model="universal-3-pro,universal-2",