Fix Sub Level parameter steps

[kovaacs]

Summary

• expose Melody and Chord Sub Level as the four hardware levels 0 through 3
• send canonical SysEx values 0, 32, 64, and 96, including modulation
• document the 2-bit hardware encoding and reject out-of-range saved state

Evidence

• Roland MKS-7 service notes describe a four-step Sub Oscillator driven by a 2-bit D/A converter: https://archive.org/download/roland_MKS-7_SERVICE_NOTES/page/n6.jpg
• The ROM disassembly masks Sub Level to bits 5 and 6: https://github.com/handsthatstrike/RolandMKS7/blob/42f7577f60265e42867dfdda0c83922650bc846a/MKS7main.asm#L3150-L3162

Compatibility

Previous saved states with Sub Level values above 3 are intentionally rejected rather than migrated.

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --all-targets --locked
• scripts/validate-bundle.sh (91 passed, 0 failed, 0 warnings, 27 skipped)

Physical hardware validation was not performed for this change.