feat: CPA/TCPA collision alerts and live track stats

CPA/TCPA alerts:
- CpaAlert data class with severity (CAUTION/WARNING/DANGER) and label()
- CpaThresholds: 0.5/1.0/2.0 nm, max TCPA 30 min
- MainViewModel.updateCpaAlerts() runs on every GPS update and AIS
  sentence; skips stationary contacts; filters TCPA ≤ 0 or > 30 min;
  sorts by TCPA ascending
- SafetyFragment: new COLLISION AVOIDANCE card shows live alert list,
  color-coded by severity; clears to "No traffic alerts" when safe
- MainActivity: observes cpaAlerts → safetyFragment.updateCpaAlerts()

Track stats:
- TrackStats data class: distanceNm, durationMs, avgSogKnots,
  durationFormatted (m:ss / h:mm:ss)
- TrackRepository.haversineNm() + computeStats() using point timestamps
- MainViewModel exposes trackStats StateFlow, cleared on stopTrack()
- activity_main: dark pill overlay bar (tv_track_stats) above map
- MainActivity: shows "● REC  0.00nm  m:ss  x.xkt avg" while recording

Tests: 95 total, all GREEN (+16 new — 9 TrackStats, 7 CpaAlert)

https://claude.ai/code/session_011h2dXbgXg3PesQMmQUNTCW

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