Promote reusable framework runtime building blocks

[0xShug0]

Summary

• promote reusable cached decode and Mimi codec runtime pieces from preview model work
• add framework MIDI file writing plus CLI artifact output support for .mid payloads
• add NormConfig::preserve_input_layout for layout-sensitive norm callsites
• add CPU-specific SDPA lowering (ExplicitCpuPerHead) and optional Qwen LM-head input cast type
• add a framework MIDI unit test with exact MIDI byte coverage

Validation

• cmake --build build/debug -j 16 --target audiocpp_cli
• cmake --build build/debug -j 16 --target midi_file_test
• ./build/debug/bin/midi_file_test

This PR intentionally excludes PersonaPlex, MuScriptor, NeuTTS, and DramaBox release files so reusable framework pieces can land first.