- Change RunSynthesis signature to accept injected config map (D-10)
- Replace hardcoded synth.ClassFreqConfigs with passed-in freqCfgs
- Update all three RunSynthesis calls in encode tests to pass synth.ClassFreqConfigs
- TestMP3Valid: synthetic snapshots through full pipeline, ffprobe validation
- TestZeroPacketError: empty slice and zero-count snapshots return error, no file created
- TestEncodeMP3DirectBytes: direct frame encoding validated by ffprobe