FFPlay strict decoder requires complete FLAC frames, unlike VLC which is more tolerant.
The previous 512-byte buffer was cutting frames mid-stream, causing sync errors.
Changes:
- Add find_last_flac_frame_boundary() to detect FLAC sync codes (0xFF 0xF8/0xF9)
- Use 16KB read buffer + accumulator to ensure frame-aligned broadcasting
- Only broadcast when we have 4KB+ of complete frames for efficiency
- Use split_off() and mem::replace() for zero-copy buffer management
- Send remaining data on EOF to avoid data loss
This fixes the "invalid sync code" and "invalid frame header" errors in FFPlay
while maintaining compatibility with VLC and other players.
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