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

1 files changed, 19 insertions, 0 deletions

diff --git a/android-app/app/src/main/res/layout/activity_main.xml b/android-app/app/src/main/res/layout/activity_main.xml
index feeb43d..a99a8cb 100644
--- a/android-app/app/src/main/res/layout/activity_main.xml
+++ b/android-app/app/src/main/res/layout/activity_main.xml
@@ -32,6 +32,25 @@
             android:visibility="gone"
             android:background="?attr/colorSurface" />
 
+        <!-- Track stats bar — shown only while recording -->
+        <TextView
+            android:id="@+id/tv_track_stats"
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:layout_gravity="top|center_horizontal"
+            android:layout_marginTop="12dp"
+            android:paddingHorizontal="14dp"
+            android:paddingVertical="6dp"
+            android:background="#CC000000"
+            android:textColor="#FFFFFFFF"
+            android:textSize="13sp"
+            android:fontFamily="monospace"
+            android:text=""
+            android:visibility="gone"
+            app:layout_constraintTop_toTopOf="parent"
+            app:layout_constraintStart_toStartOf="parent"
+            app:layout_constraintEnd_toEndOf="parent" />
+
     </androidx.constraintlayout.widget.ConstraintLayout>
 
     <!-- Collapsible Instrument Bottom Sheet -->