Add config validation for parent-child distances

Added validation in config_loader to prevent bodies from starting too close
to their parent bodies (distance must be >= parent.radius + body.radius).

Changes:
- src/config_loader.cpp: Add parent-child distance validation before initialization
- tests/configs/earth_mars_simple.toml: Fix spacecraft position to LEO altitude
  (Earth position + 6.571e6 m offset = 1.49606571e11 m)
- tests/test_invalid_parent_assignment.cpp: Update test 3 to use radius-based validation
- docs/mission_planning.md: Add TODO about spacecraft config pattern changes

Test results:
- Test 1: ✅ PASS (Earth no longer becomes child of spacecraft)
- Test 2: ✅ PASS (Mass hierarchy preserved)
- Test 3: ✅ PASS (Spacecraft at valid LEO position)
- Test 4: ❌ FAIL (mutual SOI bug, as expected)

This prevents the spacecraft initialization bug where Earth would incorrectly
become a child of the spacecraft due to distance=0.

docs/mission_planning.md

@@ -265,6 +265,19 @@ Spacecraft may not enter Mars SOI due to:
 
 ### Immediate Next Steps
 
+**TODO: Spacecraft Config Pattern Changes**
+
+Spacecraft now requires full position specification in config files:
+- Removed placeholder pattern that expected `initialize_spacecraft_leo()` to set position at runtime
+- Position must be: parent position + orbital altitude offset
+- Example: Earth LEO at 200km = Earth radius (6.371e6 m) + 200,000 m = 6.571e6 m offset
+- Velocity is calculated by `initialize_bodies()` using `semi_major_axis` parameter
+
+**Impact on `initialize_spacecraft_leo()`:**
+- Currently sets position relative to parent
+- May need to be deprecated or updated to validate/override config-based positions
+- Consider keeping function for dynamic scenarios (e.g., multi-burn missions)
+
 #### 1. Config Format Improvements
 - Support Earth-relative position specification (e.g., `{ altitude_km = 200.0 }`)
 - Support Earth-relative orbit specification (e.g., `{ orbit_type = "circular" }`)

src/config_loader.cpp

@@ -135,6 +135,26 @@ bool load_system_config(SimulationState* sim, const char* filepath) {
         }
     }
 
+    // Validate parent-child distances
+    for (int i = 0; i < body_count; i++) {
+        if (sim->bodies[i].parent_index >= 0) {
+            CelestialBody* body = &sim->bodies[i];
+            CelestialBody* parent = &sim->bodies[body->parent_index];
+
+            double distance = vec3_distance(body->position, parent->position);
+            double min_distance = parent->radius + body->radius;
+
+            if (distance < min_distance) {
+                printf("Error: Body '%s' (index %d) too close to parent '%s' (index %d)\n",
+                       body->name, i, parent->name, body->parent_index);
+                printf("  Distance: %.2e m\n", distance);
+                printf("  Minimum required: %.2e m (parent radius + body radius)\n", min_distance);
+                toml_free(result);
+                return false;
+            }
+        }
+    }
+
     sim->body_count = body_count;
     toml_free(result);
 

tests/configs/earth_mars_simple.toml

@@ -32,11 +32,12 @@ semi_major_axis = 2.279e11
 name = "Spacecraft"
 mass = 1.0
 radius = 1000.0
-# Position and velocity will be initialized at runtime for LEO orbit
-position = { x = 1.496e11, y = 0.0, z = 0.0 }
+# Position at LEO altitude: Earth position + (Earth radius + 200km altitude)
+# Earth: 1.496e11, LEO offset: 6.571e6, Total: 1.49606571e11
+position = { x = 1.49606571e11, y = 0.0, z = 0.0 }
 velocity = { x = 0.0, y = 0.0, z = 0.0 }
 parent_index = 1
 color = { r = 1.0, g = 0.0, b = 0.5 }
 eccentricity = 0.0
-# Semi-major axis will be: Earth radius + 200km
-semi_major_axis = 6.571e6  # Placeholder, will be set during initialization
+# Semi-major axis: Earth radius + 200km LEO altitude
+semi_major_axis = 6.571e6

tests/test_invalid_parent_assignment.cpp

@@ -99,12 +99,16 @@ TEST_CASE("Invalid parent: detect placeholder config values",
     Vec3 earth_pos = sim->bodies[EARTH_IDX].position;
 
     double distance = vec3_distance(craft_pos, earth_pos);
+    double min_distance = sim->bodies[EARTH_IDX].radius + sim->bodies[SPACECRAFT_IDX].radius;
 
     INFO("Distance Earth <-> Spacecraft: " << distance << " m");
     INFO("Earth position: (" << earth_pos.x << ", " << earth_pos.y << ", " << earth_pos.z << ")");
     INFO("Spacecraft position: (" << craft_pos.x << ", " << craft_pos.y << ", " << craft_pos.z << ")");
+    INFO("Earth radius: " << sim->bodies[EARTH_IDX].radius << " m");
+    INFO("Spacecraft radius: " << sim->bodies[SPACECRAFT_IDX].radius << " m");
+    INFO("Minimum required: " << min_distance << " m (parent radius + body radius)");
 
-    REQUIRE(distance > 100000.0);
+    REQUIRE(distance >= min_distance);
 
     destroy_simulation(sim);
 }