TTS-022: Add Audio File Saving to BaseProvider

[murdore]

Summary

Add helper method to BaseProvider for saving TTS audio buffers to files.

Technical Details

• File(s): src/lib/core/baseProvider.ts
• Effort: 2h

Acceptance Criteria

• [ ] Implemented saveAudioToFile() helper method
• [ ] Accept TTSResult and output path
• [ ] Auto-generate filename if path is directory
• [ ] Validate file extension matches audio format
• [ ] Use fs.promises.writeFile for async writing
• [ ] Return saved file path
• [ ] Error handling for write failures
• [ ] Create parent directories if they don't exist
• [ ] Document in JSDoc comments

Dependencies

• Depends on: TTS-001, TTS-020
• Blocks: TTS-024

Priority: medium | Effort: 2h | Complexity: simple