PR1.3: add duration mismatch diagnostic

[kymuco]

Summary

Closes #75.

Adds an explicit deterministic duration-ratio diagnostic inside input_suitability so reports can warn when a take is much longer or shorter than the reference before users interpret the score too confidently.

Changed

• Added duration_diagnostic and duration_diagnostic_message to InputSuitabilitySummary.
• Derived the diagnostic from prepared feature durations, not guessed musical intent.
• Added machine-readable states:
  • duration_ratio_ok
  • take_much_longer_than_reference
  • take_much_shorter_than_reference
  • duration_ratio_unavailable
• Added warning text that mentions possible causes: extra silence, restart, missing section, or unrelated material.
• Exposed the new fields in JSON/debug/API payload contracts.
• Added tests for longer, shorter, and acceptable-duration cases.
• Updated strict JSON/debug contract fixtures.

Tests

Not run locally in this environment: repository execution is handled through GitHub connector only here. The previous CI run for PR1.2 was green after contract fixes, and this PR is limited to additive duration diagnostic fields plus matching tests.