Bug: deterministic non-incremental vs incremental divergence at token 50 (encoder path mismatch)

[artuskg]

Title

Bug: deterministic non-incremental vs incremental divergence at token 50 (encoder path mismatch)

What this PR adds

• Adds a focused bug dossier with reproducible commands and artifacts:
  • docs/bug_reports/encoder_divergence_nonincremental_vs_incremental.md
• Adds original-author-commit reproduction artifacts proving issue predates local changes:
  • perf/audio_runs/commit-compare-e6d193e-20260209T000000Z/comparison.json
  • perf/audio_runs/commit-compare-e6d193e-20s-20260209T000000Z/comparison.json
• Adds a committed 20s fixture used by the report's token-50 claim:
  • perf/reference_audio/Paul_Solt_Ideating-and-developing-with-ChatGPT-Pro_mono_16k_20s.wav

Problem statement

On the same model/audio/settings, non-incremental file decode (generate.py with model.encode) diverges from incremental decode (encode_step) at output token index 50.
This index is documented against the included 20s fixture (and may differ for different audio).

Repro

See:

• docs/bug_reports/encoder_divergence_nonincremental_vs_incremental.md

Key evidence

• First divergence index pinned at 50 across decoder sliding_window sweep (128..16384).
• Mel parity and conv parity hold through early region; first conv mismatch occurs later (pos 134).
• Divergence localizes to encoder transformer full-vs-cached history path (layer 0 with history present).
• Same signature reproduced at original-author commit e6d193e85e84e30f26e370c66973ce287b8a9d57.

Why this matters

This causes non-incremental transcription quality collapse to PAD-heavy/special-token-heavy output relative to incremental path, limiting usable clip length for file transcription.

Proposed next step for maintainers

Treat incremental encoder path as correctness baseline and enforce full-vs-incremental equivalence checks while investigating encoder full-sequence vs cached-history attention semantics.