PR1.4: add start-region input suitability diagnostic

[kymuco]

Summary

Closes #76.

Adds a deterministic start-region diagnostic to input_suitability so real recordings can warn when the take may not begin cleanly relative to the reference.

Changed

• Added start-region fields to InputSuitabilitySummary:
  • reference_activity_start_s
  • take_activity_start_s
  • start_offset_s
  • leading_noise_duration_s
  • start_diagnostic
  • start_diagnostic_message
• Added machine-readable start diagnostics:
  • start_region_ok
  • take_activity_starts_late
  • take_leading_noise_before_activity
  • start_region_unavailable
• Derived start evidence from prepared FeatureBundle data:
  • first voiced frame;
  • first onset;
  • energy fallback only when musical start evidence is unavailable;
  • pre-activity energy for leading-noise evidence.
• Kept warning wording careful with “may indicate”, “may contain”, and “possible causes”.
• Exposed the diagnostic through JSON/debug/API payload contracts.
• Added tests for normal start, delayed take start, and noisy leading start.
• No scoring changes.

Tests

Not run locally in this environment: repository execution is handled through GitHub connector only here. The patch updates deterministic diagnostics and matching unit/contract tests.