Fix UnicodeDecodeError from subprocess stderr and set MP3 format in workflow SaveAudio nodes

[Copilot]

After successful generation, dit-vae writes non-UTF-8 binary data to stderr, causing Python's subprocess.run(..., text=True) to raise a UnicodeDecodeError. Additionally, all workflow examples left SaveAudio at ComfyUI's default FLAC format rather than MP3, which is what acestep-cpp produces.

Changes

• nodes.py: Replace text=True with encoding='utf-8', errors='replace' on both subprocess.run calls (ace-qwen3 and dit-vae). Invalid bytes are replaced instead of raising, while stderr text is still captured for error reporting.

# Before
lm_result = subprocess.run(lm_cmd, capture_output=True, text=True, cwd=tmpdir)

# After
lm_result = subprocess.run(lm_cmd, capture_output=True, encoding='utf-8', errors='replace', cwd=tmpdir)

• workflow-examples/*.json: Add "mp3" as the second widgets_values entry on every SaveAudio node across all four workflow files, overriding ComfyUI's FLAC default.

• tests/test_workflows.py: Add test_save_audio_format_is_mp3 to assert that SaveAudio nodes explicitly declare "mp3" format.

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