Adds support for rf64 as a distinct file type option while ensuring RF64 files use the correct .wav file extension.

Problem

When running svar -t webm (or any invalid file type), the error message incorrectly showed "wav" twice:

error: Unknown output file type {raw, mp3, wav, wav, ogg}: webm

Additionally, users could not specify rf64 as a file type since both WRITER_TYPE_WAV and WRITER_TYPE_RF64 mapped to the same "wav" string.

Root Cause

Both WRITER_TYPE_WAV and WRITER_TYPE_RF64 enum values mapped to the same string "wav" via the writer_type_to_string() function. This caused two issues:

1. The error message displayed "wav" twice when listing valid file types
2. Users could not specify "rf64" as a file type option

Changes Made

• Modified writer_type_to_string() in src/writer.c to return unique identifiers:
  • WRITER_TYPE_WAV returns "wav"
  • WRITER_TYPE_RF64 returns "rf64"
• Created new writer_type_to_extension() function to handle file extensions separately:
  • Both WRITER_TYPE_WAV and WRITER_TYPE_RF64 return "wav" as the file extension
• Updated src/recorder.c to use writer_type_to_extension() for file naming
• Added function declaration to src/writer.h

Result

The error message now correctly displays both options:

error: Unknown output file type {raw, mp3, wav, rf64, ogg}: webm

Users can now specify either -t wav or -t rf64, and RF64 files will be created with the correct .wav file extension.

Testing

• ✅ Build completes successfully with all audio codec support enabled
• ✅ Verified error message shows both "wav" and "rf64" as separate options
• ✅ Confirmed RF64 files use .wav extension (not .rf64)
• ✅ Updated .gitignore to exclude build artifacts

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