fix: Incorrect type annotations — use Optional[T] instead of T for nullable defaults

Bug Report

Several parameters use bare type annotations with None as default value (e.g., int = None, str = None), which is incorrect. These should use Optional[T] = None to properly indicate nullable types per PEP 484.

Affected locations

1. src/voxcpm/modules/minicpm4/config.py (line 29):

   • kv_channels: int = None → kv_channels: Optional[int] = None

2. src/voxcpm/modules/locdit/local_dit.py (line 30):

   • out_dim: int = None → out_dim: Optional[int] = None

3. src/voxcpm/core.py (lines 109, 183–185):

   • cache_dir: str = None → cache_dir: Optional[str] = None
   • prompt_wav_path: str = None → prompt_wav_path: Optional[str] = None
   • prompt_text: str = None → prompt_text: Optional[str] = None
   • reference_wav_path: str = None → reference_wav_path: Optional[str] = None

Expected behavior

All parameters that default to None should be annotated with Optional[T] for correct type checking.

Read the affected annotations in src/voxcpm/modules/minicpm4/config.py, src/voxcpm/modules/locdit/local_dit.py, and src/voxcpm/core.py. Update each parameter that defaults to None to use Optional[T], then verify that all listed nullable parameters are annotated consistently for type checking.