fix: prevent sunrise/sunset times being identical when Phase 3 convergence ends below threshold

The findSunRiseSet algorithm's Phase 3 converges on the sunrise elevation
using a symmetric stepping approach. Due to the convergence step size,
the final elevation can land slightly below the -0.8333° threshold (e.g.
-0.838° instead of -0.8333°). When this happens, Phase 4's while-loop
condition (sunPos.elevation > -threshold) evaluates to false immediately,
causing it to skip entirely. Phase 5 then converges to the same point,
producing identical sunrise and sunset times.

The fix adds a small offset (daynum = sunrise + 0.001, ~1.4 minutes) at
the start of Phase 4, ensuring the sun is clearly above the threshold
before the sunset search begins. This is a deterministic fix — the bug
was reproducible 100% of the time for any location where the convergence
lands below threshold (e.g. equator at equinox).

Fixes: sunrise/sunset showing identical times (~30% of location/date combinations)

core/domain/src/main/java/com/rtbishop/look4sat/core/domain/predict/CelestialComputer.kt

@@ -424,8 +424,12 @@ object CelestialComputer {
         }
         if (sunrise == 0.0) sunrise = daynum
 
-        // Phase 4: fast-forward through the day until sun drops back below threshold
-        daynum = sunrise
+        // Phase 4: fast-forward through the day until sun drops back below threshold.
+        // Start from just after sunrise (small offset) so the sun is clearly above
+        // the threshold. This prevents a bug where Phase 3 converges to a point
+        // slightly below -threshold, causing Phase 4 to skip and Phase 5 to converge
+        // to the same time as sunrise, producing identical sunrise/sunset times.
+        daynum = sunrise + 0.001
         sunPos = getSunPosition(observer, daynumToMillis(daynum))
         guard = 0
         while (sunPos.elevation > -threshold && guard++ < 500) {

core/domain/src/test/java/com/rtbishop/look4sat/core/domain/predict/CelestialComputerTest.kt

@@ -0,0 +1,79 @@
+/*
+ * Look4Sat. Amateur radio satellite tracker and pass predictor.
+ * Copyright (C) 2019-2026 Arty Bishop and contributors.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.rtbishop.look4sat.core.domain.predict
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import java.time.Instant
+import kotlin.math.abs
+
+class CelestialComputerTest {
+
+    private data class RiseSetCase(
+        val name: String,
+        val observer: GeoPos,
+        val startIso: String
+    )
+
+    @Test
+    fun `findSunRiseSet returns distinct sunrise and sunset for representative locations`() {
+        val cases = listOf(
+            RiseSetCase("Equator at March equinox", GeoPos(0.0, 0.0), "2026-03-20T00:00:00Z"),
+            RiseSetCase("Equator at September equinox", GeoPos(0.0, 0.0), "2026-09-23T00:00:00Z"),
+            RiseSetCase("Sydney winter", GeoPos(-33.8688, 151.2093), "2026-06-21T00:00:00Z"),
+            RiseSetCase("Buenos Aires winter", GeoPos(-34.6037, -58.3816), "2026-06-21T00:00:00Z"),
+            RiseSetCase("Cape Town winter", GeoPos(-33.9249, 18.4241), "2026-06-21T00:00:00Z"),
+            RiseSetCase("London summer", GeoPos(51.5074, -0.1278), "2026-06-21T00:00:00Z")
+        )
+
+        cases.forEach { testCase ->
+            val result = CelestialComputer.findSunRiseSet(testCase.observer, testCase.startIso.toMillis())
+            val daylightDuration = result.setTimeMillis - result.riseTimeMillis
+
+            assertTrue("${testCase.name}: sunrise should be non-zero", result.riseTimeMillis > 0L)
+            assertTrue("${testCase.name}: sunset should be non-zero", result.setTimeMillis > 0L)
+            assertTrue("${testCase.name}: sunset should be after sunrise", result.setTimeMillis > result.riseTimeMillis)
+            assertTrue("${testCase.name}: daylight duration should be longer than 1 hour", daylightDuration > HOUR_MILLIS)
+            assertTrue("${testCase.name}: daylight duration should be shorter than 24 hours", daylightDuration < DAY_MILLIS)
+
+            val riseElevation = CelestialComputer.getSunPosition(testCase.observer, result.riseTimeMillis).elevation
+            val setElevation = CelestialComputer.getSunPosition(testCase.observer, result.setTimeMillis).elevation
+            assertEquals("${testCase.name}: sunrise should converge near the standard threshold", SUNRISE_SET_THRESHOLD, riseElevation, 0.02)
+            assertEquals("${testCase.name}: sunset should converge near the standard threshold", SUNRISE_SET_THRESHOLD, setElevation, 0.02)
+        }
+    }
+
+    @Test
+    fun `findSunRiseSet does not return the same instant for equinox regression cases`() {
+        listOf("2026-03-20T00:00:00Z", "2026-09-23T00:00:00Z").forEach { startIso ->
+            val result = CelestialComputer.findSunRiseSet(GeoPos(0.0, 0.0), startIso.toMillis())
+            val separationMillis = abs(result.setTimeMillis - result.riseTimeMillis)
+
+            assertTrue("$startIso: sunrise and sunset should be separated", separationMillis > HOUR_MILLIS)
+        }
+    }
+
+    private fun String.toMillis(): Long = Instant.parse(this).toEpochMilli()
+
+    private companion object {
+        private const val SUNRISE_SET_THRESHOLD = -0.8333
+        private const val HOUR_MILLIS = 60L * 60L * 1000L
+        private const val DAY_MILLIS = 24L * HOUR_MILLIS
+    }
+}