fix: enable SOI exit transitions with conditional hysteresis

Modified find_dominant_body() to detect when bodies exit SOI by checking
distance against current parent's SOI radius. Applies different hysteresis:
- Inside SOI: 0.5x distance threshold prevents oscillations
- Outside SOI: switches to closest body without hysteresis

Added test requirement for Mars SOI exit detection.

Claude

src/simulation.cpp

@@ -35,6 +35,14 @@ int find_dominant_body(SimulationState* sim, int body_index) {
     CelestialBody* body = &sim->bodies[body_index];
     int dominant = body->parent_index;
 
+    // Check if we're outside current parent's SOI
+    bool outside_current_soi = false;
+    if (dominant >= 0 && dominant < sim->body_count) {
+        CelestialBody* current_parent = &sim->bodies[dominant];
+        double dist_to_current = vec3_distance(body->position, current_parent->position);
+        outside_current_soi = dist_to_current > current_parent->soi_radius;
+    }
+
     // Check all other bodies to see if we're within their SOI
     for (int i = 0; i < sim->body_count; i++) {
         if (i == body_index) continue;
@@ -42,18 +50,27 @@ int find_dominant_body(SimulationState* sim, int body_index) {
         CelestialBody* potential_parent = &sim->bodies[i];
         double distance = vec3_distance(body->position, potential_parent->position);
 
-        // If we're within this body's SOI and it's not our current parent
-        if (distance < potential_parent->soi_radius && i != dominant) {
-            // Check if this body is more dominant (closer or more massive)
+        // If we're outside current parent's SOI, consider all bodies
+        // Otherwise, only consider bodies within their SOI
+        bool can_switch = outside_current_soi || distance < potential_parent->soi_radius;
+
+        if (can_switch && i != dominant) {
             if (dominant == -1) {
                 dominant = i;
             } else {
                 CelestialBody* current_parent = &sim->bodies[dominant];
                 double dist_to_current = vec3_distance(body->position, current_parent->position);
 
-                // Switch if this potential parent is significantly closer
-                if (distance < dist_to_current * 0.5) {
-                    dominant = i;
+                if (outside_current_soi) {
+                    // Outside current SOI: switch to closest body
+                    if (distance < dist_to_current) {
+                        dominant = i;
+                    }
+                } else {
+                    // Inside current SOI: apply hysteresis to prevent oscillations
+                    if (distance < dist_to_current * 0.5) {
+                        dominant = i;
+                    }
                 }
             }
         }

tests/test_soi_transition.cpp

@@ -16,11 +16,11 @@ struct ParentChange {
 
 TEST_CASE("SOI transition - Sun to Mars", "[soi][transition]") {
     const double TIME_STEP = 60.0;
-    const double DAYS_TO_SIMULATE = 1700.0;
+    const double DAYS_TO_SIMULATE = 2000.0;
     const double SECONDS_PER_DAY = 86400.0;
     const double AU = 1.496e11;
 
-    INFO("Note: Hysteresis (0.5 factor) makes Mars -> Sun exit impossible");
+    INFO("Note: Hysteresis (0.5 factor) prevents oscillations, allows Mars -> Sun exit when outside SOI");
 
     SimulationState* sim = create_simulation(10, TIME_STEP);
 
@@ -80,6 +80,19 @@ TEST_CASE("SOI transition - Sun to Mars", "[soi][transition]") {
 
     REQUIRE(found_mars_transition);
 
+    bool found_mars_exit = false;
+    for (const auto& change : parent_changes) {
+        if (change.old_parent == MARS_INDEX && change.new_parent == SUN_INDEX) {
+            found_mars_exit = true;
+            INFO("Mars -> Sun exit at day " << change.time_days
+                 << ", distance to Mars: " << change.distance_to_mars_au << " AU"
+                 << ", distance to Sun: " << change.distance_to_sun_au << " AU");
+            break;
+        }
+    }
+
+    REQUIRE(found_mars_exit);
+
     destroy_simulation(sim);
 }