Add test cases for prograde burn at periapsis

Documents a bug where true anomaly triggers set to 0 (periapsis) don't
execute because the spacecraft moves past periapsis before the trigger
check happens. The trigger check occurs after physics propagation in the
simulation update order.

- test_periapsis_burn.cpp: Three test cases demonstrating expected behavior
  1. Verify periapsis distance is preserved after prograde burn (fails - shows bug)
  2. Verify apoapsis raises and periapsis stays same (skipped - depends on fix)
  3. Verify burn location equals new periapsis (skipped - depends on fix)

- test_periapsis_burn.toml: Test configuration with elliptical orbit satellite
  starting at periapsis with a true anomaly trigger at 0

tests/test_periapsis_burn.cpp

@@ -0,0 +1,123 @@
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/matchers/catch_matchers_floating_point.hpp>
+#include "../src/physics.h"
+#include "../src/simulation.h"
+#include "../src/spacecraft.h"
+#include "../src/maneuver.h"
+#include "../src/config_loader.h"
+#include "../src/orbital_mechanics.h"
+#include <cmath>
+
+// This test documents a bug: when a true anomaly trigger is set to 0 (periapsis),
+// the maneuver doesn't execute because the spacecraft moves past periapsis before
+// the trigger check happens (trigger check occurs after physics update).
+// The expected behavior is that the maneuver should execute when at periapsis.
+TEST_CASE("Prograde burn at periapsis preserves periapsis distance", "[maneuver][periapsis][bug]") {
+    const double TIME_STEP = 60.0;
+    SimulationState* sim = create_simulation(10, 10, 100, TIME_STEP);
+
+    REQUIRE(load_system_config(sim, "tests/test_periapsis_burn.toml"));
+
+    Spacecraft* craft = &sim->spacecraft[0];
+
+    // Record initial state (at periapsis)
+    double initial_radius = vec3_magnitude(craft->local_position);
+    double initial_sma = craft->orbit.semi_major_axis;
+    double initial_ecc = craft->orbit.eccentricity;
+    double initial_periapsis = initial_sma * (1.0 - initial_ecc);
+    double initial_velocity = vec3_magnitude(craft->local_velocity);
+
+    INFO("Initial state:");
+    INFO("  Radius: " << initial_radius << " (should equal periapsis: " << initial_periapsis << ")");
+    INFO("  True anomaly: " << craft->orbit.true_anomaly);
+    INFO("  Velocity: " << initial_velocity);
+    INFO("  Maneuver trigger: true_anomaly = " << sim->maneuvers[0].trigger_value);
+
+    // Execute one step
+    update_simulation(sim);
+
+    // Check if maneuver executed
+    INFO("After 1 step:");
+    INFO("  Maneuver executed: " << sim->maneuvers[0].executed);
+    INFO("  Final radius: " << vec3_magnitude(craft->local_position));
+    INFO("  Final velocity: " << vec3_magnitude(craft->local_velocity));
+
+    // The maneuver should have executed since we started at periapsis
+    // This assertion will fail due to the bug - the trigger check happens
+    // after physics moves the spacecraft past periapsis
+    REQUIRE(sim->maneuvers[0].executed);
+
+    // If the maneuver executed, verify the physics:
+    double final_sma = craft->orbit.semi_major_axis;
+    double final_ecc = craft->orbit.eccentricity;
+    double final_periapsis = final_sma * (1.0 - final_ecc);
+    double final_velocity = vec3_magnitude(craft->local_velocity);
+
+    // Periapsis distance should be preserved
+    REQUIRE_THAT(final_periapsis, Catch::Matchers::WithinAbs(initial_periapsis, 1.0));
+
+    // Semi-major axis and velocity should increase
+    REQUIRE(final_sma > initial_sma);
+    REQUIRE(final_velocity > initial_velocity);
+
+    destroy_simulation(sim);
+}
+
+TEST_CASE("Prograde burn at periapsis raises apoapsis not periapsis", "[maneuver][periapsis][apoapsis]") {
+    const double TIME_STEP = 60.0;
+    SimulationState* sim = create_simulation(10, 10, 100, TIME_STEP);
+
+    REQUIRE(load_system_config(sim, "tests/test_periapsis_burn.toml"));
+
+    Spacecraft* craft = &sim->spacecraft[0];
+
+    double initial_periapsis = craft->orbit.semi_major_axis * (1.0 - craft->orbit.eccentricity);
+    double initial_apoapsis = craft->orbit.semi_major_axis * (1.0 + craft->orbit.eccentricity);
+
+    update_simulation(sim);
+
+    // Skip verification if bug not fixed
+    if (!sim->maneuvers[0].executed) {
+        SKIP("Maneuver didn't execute - known bug");
+    }
+
+    double final_sma = craft->orbit.semi_major_axis;
+    double final_ecc = craft->orbit.eccentricity;
+    double final_periapsis = final_sma * (1.0 - final_ecc);
+    double final_apoapsis = final_sma * (1.0 + final_ecc);
+
+    INFO("Initial: peri=" << initial_periapsis << " apo=" << initial_apoapsis);
+    INFO("Final:   peri=" << final_periapsis << " apo=" << final_apoapsis);
+
+    REQUIRE(final_apoapsis > initial_apoapsis);
+    REQUIRE_THAT(final_periapsis, Catch::Matchers::WithinAbs(initial_periapsis, 1.0));
+
+    destroy_simulation(sim);
+}
+
+TEST_CASE("Burn location equals new periapsis after prograde burn", "[maneuver][periapsis][location]") {
+    const double TIME_STEP = 60.0;
+    SimulationState* sim = create_simulation(10, 10, 100, TIME_STEP);
+
+    REQUIRE(load_system_config(sim, "tests/test_periapsis_burn.toml"));
+
+    Spacecraft* craft = &sim->spacecraft[0];
+
+    double burn_radius = vec3_magnitude(craft->local_position);
+
+    update_simulation(sim);
+
+    // Skip verification if bug not fixed
+    if (!sim->maneuvers[0].executed) {
+        SKIP("Maneuver didn't execute - known bug");
+    }
+
+    double final_periapsis = craft->orbit.semi_major_axis * (1.0 - craft->orbit.eccentricity);
+
+    INFO("Burn radius: " << burn_radius);
+    INFO("Final periapsis: " << final_periapsis);
+
+    REQUIRE_THAT(burn_radius, Catch::Matchers::WithinAbs(final_periapsis, 1.0));
+
+    destroy_simulation(sim);
+}

tests/test_periapsis_burn.toml

@@ -0,0 +1,47 @@
+# Test Configuration: Prograde Burn at Periapsis
+# Tests that periapsis distance is preserved during prograde burn
+
+[[bodies]]
+name = "Sun"
+mass = 1.989e30
+radius = 6.96e8
+parent_index = -1
+color = { r = 1.0, g = 1.0, b = 0.0 }
+orbit = {
+    semi_major_axis = 0.0,
+    eccentricity = 0.0,
+    true_anomaly = 0.0
+}
+
+[[bodies]]
+name = "Earth"
+mass = 5.972e24
+radius = 6.371e6
+parent_index = 0
+color = { r = 0.0, g = 0.5, b = 1.0 }
+orbit = {
+    semi_major_axis = 1.496e11,
+    eccentricity = 0.0,
+    true_anomaly = 0.0
+}
+
+[[spacecraft]]
+name = "TestSatellite"
+mass = 1000.0
+parent_index = 1
+# Start at periapsis of an elliptical orbit
+# a = 10,000 km, e = 0.3
+# periapsis = 7,000 km, apoapsis = 13,000 km
+orbit = {
+    semi_major_axis = 1.0371e7,
+    eccentricity = 0.3,
+    true_anomaly = 0.0
+}
+
+[[maneuvers]]
+name = "periapsis_prograde_burn"
+spacecraft_name = "TestSatellite"
+trigger_type = "true_anomaly"
+trigger_value = 0.0
+direction = "prograde"
+delta_v = 500.0