Function validate 

pub fn validate(score: &Score) -> ValidationReport

Check every voice in every measure for structural correctness.

Checks performed:

• Errors: beat-count mismatch, out-of-range pitch.
• Warnings: incomplete bar (underfull voice), overlapping volta brackets, empty parts, duplicate rehearsal marks.

Multi-rest placeholder measures and empty voices are skipped. Percussion parts (MIDI channel 9) are exempt from pitch-range checks.