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

2 files changed, 157 insertions, 0 deletions

diff --git a/test-runner/src/test/kotlin/org/terst/nav/ais/CpaAlertTest.kt b/test-runner/src/test/kotlin/org/terst/nav/ais/CpaAlertTest.kt
new file mode 100644
index 0000000..3effb24
--- /dev/null
+++ b/test-runner/src/test/kotlin/org/terst/nav/ais/CpaAlertTest.kt
@@ -0,0 +1,78 @@
+package org.terst.nav.ais
+
+import org.junit.Assert.*
+import org.junit.Test
+
+class CpaAlertTest {
+
+    // --- CpaCalculator ---
+
+    @Test
+    fun `head-on convergence produces small CPA and positive TCPA`() {
+        // Own vessel heading north at 10 kt; target heading south at 10 kt, 1 nm ahead
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 10.0, ownCog = 0.0,
+            tgtLat = 1.0 / 60.0, tgtLon = 0.0, tgtSog = 10.0, tgtCog = 180.0
+        )
+        assertTrue("CPA should be near zero, got $cpa", cpa < 0.1)
+        assertTrue("TCPA should be positive, got $tcpa", tcpa > 0)
+        assertTrue("TCPA should be < 5 min, got $tcpa", tcpa < 5.0)
+    }
+
+    @Test
+    fun `diverging vessels produce negative TCPA`() {
+        // Own heading north, target heading north ahead — both going same direction with target faster
+        val (_, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 5.0, ownCog = 0.0,
+            tgtLat = 1.0 / 60.0, tgtLon = 0.0, tgtSog = 10.0, tgtCog = 0.0
+        )
+        assertTrue("TCPA should be negative for diverging vessels, got $tcpa", tcpa < 0)
+    }
+
+    @Test
+    fun `zero relative velocity returns current distance`() {
+        // Same velocity — parallel course, same speed
+        val (cpa, tcpa) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 5.0, ownCog = 90.0,
+            tgtLat = 1.0 / 60.0, tgtLon = 0.0, tgtSog = 5.0, tgtCog = 90.0
+        )
+        assertTrue("CPA should equal current distance, got $cpa", cpa > 0.9 && cpa < 1.1)
+        assertEquals(0.0, tcpa, 1e-9)
+    }
+
+    @Test
+    fun `well-separated crossing vessels produce large CPA`() {
+        // Perpendicular courses but far apart — should not alert
+        val (cpa, _) = CpaCalculator.compute(
+            ownLat = 0.0, ownLon = 0.0, ownSog = 8.0, ownCog = 0.0,
+            tgtLat = 0.0, tgtLon = 5.0, tgtSog = 8.0, tgtCog = 270.0
+        )
+        assertTrue("CPA should be > 2 nm for well-separated vessels, got $cpa", cpa > 2.0)
+    }
+
+    // --- Severity classification ---
+
+    @Test
+    fun `DANGER severity for CPA under 0_5 nm`() {
+        val alert = CpaAlert(123, "TARGET", 0.3, 10.0, CpaSeverity.DANGER)
+        assertEquals(CpaSeverity.DANGER, alert.severity)
+    }
+
+    @Test
+    fun `label formats correctly with vessel name`() {
+        val alert = CpaAlert(123456789, "STENA IMPERO", 0.8, 12.5, CpaSeverity.WARNING)
+        val lbl = alert.label()
+        assertTrue(lbl.contains("0.8"))
+        assertTrue(lbl.contains("13") || lbl.contains("12"))  // rounded min
+        assertTrue(lbl.contains("STENA"))
+    }
+
+    @Test
+    fun `label uses MMSI prefix when name is blank`() {
+        val alert = CpaAlert(987654321, "", 1.5, 20.0, CpaSeverity.CAUTION)
+        // "MMSI 987654321" is 14 chars; label() takes the first 12 -> "MMSI 9876543"
+        val lbl = alert.label()
+        assertTrue("Expected MMSI prefix in label, got: $lbl", lbl.contains("MMSI"))
+        assertTrue("Expected partial MMSI in label, got: $lbl", lbl.contains("987654"))
+    }
+}
diff --git a/test-runner/src/test/kotlin/org/terst/nav/track/TrackStatsTest.kt b/test-runner/src/test/kotlin/org/terst/nav/track/TrackStatsTest.kt
new file mode 100644
index 0000000..7bce0a2
--- /dev/null
+++ b/test-runner/src/test/kotlin/org/terst/nav/track/TrackStatsTest.kt
@@ -0,0 +1,79 @@
+package org.terst.nav.track
+
+import org.junit.Assert.*
+import org.junit.Before
+import org.junit.Test
+
+class TrackStatsTest {
+
+    private lateinit var repo: TrackRepository
+
+    private fun pt(lat: Double, lon: Double, sog: Double = 5.0, ts: Long = 0L) =
+        TrackPoint(lat, lon, sog, 0.0, 0.0, 0.0, false, ts)
+
+    @Before
+    fun setup() {
+        repo = TrackRepository()
+        repo.startTrack()
+    }
+
+    @Test
+    fun `computeStats returns null for single point`() {
+        repo.addPoint(pt(41.0, -71.0, ts = 0))
+        assertNull(repo.computeStats())
+    }
+
+    @Test
+    fun `computeStats returns null when not recording and no points`() {
+        repo.stopTrack()
+        assertNull(repo.computeStats())
+    }
+
+    @Test
+    fun `haversineNm one degree of latitude is approximately 60 nm`() {
+        val d = TrackRepository.haversineNm(0.0, 0.0, 1.0, 0.0)
+        assertEquals(60.0, d, 0.5)
+    }
+
+    @Test
+    fun `haversineNm zero distance returns zero`() {
+        assertEquals(0.0, TrackRepository.haversineNm(41.0, -71.0, 41.0, -71.0), 1e-9)
+    }
+
+    @Test
+    fun `computeStats distance accumulates across legs`() {
+        repo.addPoint(pt(0.0, 0.0, ts = 0))
+        repo.addPoint(pt(1.0, 0.0, ts = 3_600_000))  // ~60 nm north
+        repo.addPoint(pt(1.0, 1.0, ts = 7_200_000))  // ~60 nm east at 1° lat
+        val stats = repo.computeStats()!!
+        assertTrue("Expected ~120 nm, got ${stats.distanceNm}", stats.distanceNm > 100.0)
+    }
+
+    @Test
+    fun `computeStats duration is last minus first timestamp`() {
+        repo.addPoint(pt(0.0, 0.0, ts = 1_000))
+        repo.addPoint(pt(1.0, 0.0, ts = 3_601_000))
+        val stats = repo.computeStats()!!
+        assertEquals(3_600_000L, stats.durationMs)
+    }
+
+    @Test
+    fun `computeStats avgSog is mean of all point sogs`() {
+        repo.addPoint(pt(0.0, 0.0, sog = 4.0, ts = 0))
+        repo.addPoint(pt(0.0, 0.1, sog = 6.0, ts = 1000))
+        val stats = repo.computeStats()!!
+        assertEquals(5.0, stats.avgSogKnots, 0.001)
+    }
+
+    @Test
+    fun `durationFormatted shows m_ss below one hour`() {
+        val stats = TrackStats(1.0, 65_000L, 5.0)  // 1 min 5 sec
+        assertEquals("1:05", stats.durationFormatted)
+    }
+
+    @Test
+    fun `durationFormatted shows h_mm_ss above one hour`() {
+        val stats = TrackStats(1.0, 3_661_000L, 5.0)  // 1h 1m 1s
+        assertEquals("1:01:01", stats.durationFormatted)
+    }
+}