Document firmware protocol cross-check

[kovaacs]

Summary

• link the protocol reference to the pinned reverse-engineered MKS-7 main ROM revision
• document the opcode 32 parser flow and confirm that short parameter writes need no padding
• identify firmware routing labels for Melody, Chord, Bass, and Whole Mode behavior
• distinguish ROM-confirmed mechanics from hardware-confirmed audible behavior

Validation

• Formatting passes (cargo fmt --all -- --check)
• Clippy passes (cargo clippy --workspace --all-targets --locked -- -D warnings)
• Tests pass (cargo test --workspace --all-targets --locked)
• Bundle validation passes (scripts/validate-bundle.sh), or packaging and CLAP behavior are not affected
• Physical MKS-7 behavior was tested, or hardware behavior is not affected
• Documentation and third-party license output were updated if needed

Hardware Notes

Physical hardware testing was not repeated because this PR changes documentation only. Existing hardware-verified claims remain explicitly distinguished from conclusions drawn from the ROM source.