[Spec 02] Phrase-aware pause detection + filled pauses

[fwright21]

Summary

Implement phrase-aware pause detection and filled-pause handling (Spec 02) so pauses are classified with context and surfaced with timestamps.

Scope

• Classify silence as natural (between-phrase) vs unnatural (within-phrase).
• Detect leading/trailing silence separately so it does not get mislabeled as within-phrase.
• Detect filled pauses (um, uh, este, mmm, o sea, etc.) with language-aware matching.
• Add/propagate flagged_regions timestamps for UI jump-to-region support.
• Keep fallback behavior when alignment spans are unavailable.

Acceptance Criteria

• Within-phrase long pauses produce warn/fail candidates.
• Between-phrase pauses do not get penalized.
• Leading/trailing silence is tracked without false within-phrase flags.
• Filled pauses are detected with reliable timestamps.
• Results flow into suite + UI payloads.