fix: sanitize_output_stem signature + audio_sink import

Two pre-existing bugs found during test coverage analysis:

1. sanitize_output_stem() only accepted 1 arg but resolve_project_layout
   passed 2 args (name, index) via sanitize_fn parameter.
   Fix: added optional index parameter to sanitize_output_stem.

2. audio_sink.py imported get_internal_cache_path from
   abogen.infrastructure.cache which doesn't exist.
   Fix: import from abogen.utils where the function lives.

abogen/domain/audio_sink.py

@@ -54,7 +54,7 @@ def _ensure_ffmpeg() -> None:
 
 
 def _get_ffmpeg_cache_root() -> str:
-    from abogen.infrastructure.cache import get_internal_cache_path
+    from abogen.utils import get_internal_cache_path
 
     return get_internal_cache_path("ffmpeg")
 

abogen/domain/output_paths.py

@@ -76,7 +76,7 @@ def sanitize_filename_for_chapter(title: str, index: int, max_len: int = 80) ->
     return f"{index:02d}_{sanitized}"
 
 
-def sanitize_output_stem(name: str) -> str:
+def sanitize_output_stem(name: str, index: int = 0) -> str:
     base = Path(name or "").stem
     sanitized = _OUTPUT_SANITIZE_RE.sub("_", base).strip("_")
     return sanitized or "output"