refactor: metadata + markers unified in shared layer

- MarkerCollector: SRP extraction from executor (observation, not execution)
- Outro marker: executor now records outro as chapter marker
- Voice format: chapter voices [{provider, voice}], chunk voice {provider, voice}
- _finalize(): build_metadata_payload() + metadata.json + record_override_usage()
- ffmetadata: voices list → comma-separated string
- EPUB3: ChunkOverlay.voice = dict, _render_chunk_inline handles dict format
- 8 new tests: multi-speaker, ffmetadata format, EPUB3 format
- Updated existing tests for new voice format

abogen/infrastructure/exporters.py

@@ -84,9 +84,14 @@ class ExportService:
             title = chapter.get("title")
             if title:
                 lines.append(f"title={self._escape_ffmetadata_value(title)}")
-            voice = chapter.get("voice")
-            if voice:
-                lines.append(f"voice={self._escape_ffmetadata_value(voice)}")
+            voices = chapter.get("voices")
+            if voices and isinstance(voices, list):
+                voice_str = ", ".join(
+                    f"{v.get('voice', '')}@{v.get('provider', '')}"
+                    for v in voices if v.get("voice")
+                )
+                if voice_str:
+                    lines.append(f"voice={self._escape_ffmetadata_value(voice_str)}")
         
         return "\n".join(lines) + "\n"