refactor: create test_energy.cpp, move direction test from orbital_period

- New test_energy.cpp: energy conservation (10-day drift < 1e-12) and
  zero-inclination prograde direction verification
- Tighten energy assertion: replaced qualitative < 5% with WithinAbs(0.0, 1e-12)
  (actual drift ~2e-16 for 10-day simulation)
- Remove direction test from test_orbital_period.cpp (semantic separation)
- All 16 assertions across 3 test files passing

tests/test_energy.cpp

@@ -0,0 +1,60 @@
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/matchers/catch_matchers_floating_point.hpp>
+#include "../src/physics.h"
+#include "../src/simulation.h"
+#include "../src/config_loader.h"
+#include "../src/test_utilities.h"
+#include <cmath>
+
+using Catch::Matchers::WithinAbs;
+
+SCENARIO("Energy conservation in circular orbit", "[energy][sanity]") {
+    const double TIME_STEP = 60.0;
+    const double DAYS_TO_SIMULATE = 10.0;
+    const double SECONDS_PER_DAY = 86400.0;
+
+    SimulationState* sim = create_simulation(2, 0, 0, TIME_STEP);
+    REQUIRE(load_system_config(sim, "tests/test_energy.toml"));
+
+    const double initial_energy = calculate_system_total_energy(sim);
+
+    double total_time = DAYS_TO_SIMULATE * SECONDS_PER_DAY;
+    while (sim->time < total_time) {
+        update_simulation(sim);
+    }
+
+    const double final_energy = calculate_system_total_energy(sim);
+    const double energy_drift = fabs(final_energy - initial_energy) / fabs(initial_energy);
+
+    INFO("Initial energy: " << initial_energy << " J");
+    INFO("Final energy: " << final_energy << " J");
+    INFO("Relative drift: " << energy_drift << " (fraction)");
+
+    REQUIRE_THAT(energy_drift, WithinAbs(0.0, 1e-12));
+
+    destroy_simulation(sim);
+}
+
+SCENARIO("Orbit direction for zero inclination", "[direction][sanity]") {
+    const double TIME_STEP = 60.0;
+    const int STEPS = 1440; // 1 day at 60s steps
+
+    SimulationState* sim = create_simulation(2, 0, 0, TIME_STEP);
+    REQUIRE(load_system_config(sim, "tests/test_energy.toml"));
+
+    CelestialBody* sun = &sim->bodies[0];
+    CelestialBody* earth = &sim->bodies[1];
+
+    Vec3 initial_rel = vec3_sub(earth->global_position, sun->global_position);
+    const double theta_start = atan2(initial_rel.y, initial_rel.x);
+
+    for (int i = 0; i < STEPS; i++) update_simulation(sim);
+
+    Vec3 final_rel = vec3_sub(earth->global_position, sun->global_position);
+    const double delta = atan2(final_rel.y, final_rel.x) - theta_start;
+
+    INFO("Delta: " << delta << " rad");
+    REQUIRE_THAT(delta, WithinAbs(0.0172042841, 1e-6));
+
+    destroy_simulation(sim);
+}

tests/test_orbital_period.cpp

@@ -16,80 +16,39 @@ SCENARIO("Orbital period measurement with analytical propagation",
     SimulationState* sim = create_simulation(10, 0, 0, TIME_STEP);
     REQUIRE(load_system_config(sim, "tests/test_orbital_period.toml"));
 
-    SECTION("Earth completes one orbit in ~365 days") {
-        OrbitTracker* tracker = create_orbit_tracker(1);
-        CelestialBody* earth = &sim->bodies[1];
-        CelestialBody* sun = &sim->bodies[0];
-
-        const double MAX_DAYS = 400.0;
-        const double max_time = MAX_DAYS * SECONDS_PER_DAY;
-
+    auto propagate_and_check = [&](OrbitTracker* tracker, CelestialBody* body,
+                                   CelestialBody* parent, double max_time,
+                                   double expected_days) {
         while (sim->time < max_time && !tracker->orbit_completed) {
             update_simulation(sim);
-            update_orbit_tracker(tracker, earth, sun, sim->time);
+            update_orbit_tracker(tracker, body, parent, sim->time);
         }
-
         REQUIRE(tracker->orbit_completed);
 
-        double measured_days = tracker->time_at_completion / SECONDS_PER_DAY;
+        const double measured_days = tracker->time_at_completion / SECONDS_PER_DAY;
         INFO("Measured period: " << measured_days << " days");
-
-        REQUIRE_THAT(measured_days, WithinAbs(365.2105, 0.1));
+        REQUIRE_THAT(measured_days, WithinAbs(expected_days, 0.1));
 
         destroy_orbit_tracker(tracker);
+    };
+
+    SECTION("Earth completes one orbit in ~365 days") {
+        CelestialBody* earth = &sim->bodies[1];
+        CelestialBody* sun = &sim->bodies[0];
+        OrbitTracker* tracker = create_orbit_tracker(1);
+
+        const double max_time = 400.0 * SECONDS_PER_DAY;
+        propagate_and_check(tracker, earth, sun, max_time, 365.2105);
     }
 
     SECTION("Mars completes one orbit in ~671 days") {
-        OrbitTracker* tracker = create_orbit_tracker(2);
         CelestialBody* mars = &sim->bodies[2];
         CelestialBody* sun = &sim->bodies[0];
+        OrbitTracker* tracker = create_orbit_tracker(2);
 
-        const double MAX_DAYS = 750.0;
-        const double max_time = MAX_DAYS * SECONDS_PER_DAY;
-
-        while (sim->time < max_time && !tracker->orbit_completed) {
-            update_simulation(sim);
-            update_orbit_tracker(tracker, mars, sun, sim->time);
-        }
-
-        REQUIRE(tracker->orbit_completed);
-
-        double measured_days = tracker->time_at_completion / SECONDS_PER_DAY;
-        INFO("Measured period: " << measured_days << " days");
-
-        REQUIRE_THAT(measured_days, WithinAbs(670.9345, 0.1));
-
-        destroy_orbit_tracker(tracker);
+        const double max_time = 750.0 * SECONDS_PER_DAY;
+        propagate_and_check(tracker, mars, sun, max_time, 670.9345);
     }
 }
 
-SCENARIO("Orbit direction for zero inclination",
-         "[direction][sanity]") {
-    const double TIME_STEP = 60.0;
-    const double TEST_DURATION_DAYS = 1.0;
-    const int STEPS = (int)(TEST_DURATION_DAYS * 86400.0 / TIME_STEP);
-
-    SimulationState* sim = create_simulation(2, 0, 0, TIME_STEP);
-    REQUIRE(load_system_config(sim, "tests/test_energy.toml"));
-
-    CelestialBody* sun = &sim->bodies[0];
-    CelestialBody* earth = &sim->bodies[1];
 
-    Vec3 initial_rel = vec3_sub(earth->global_position, sun->global_position);
-    double theta_start = atan2(initial_rel.y, initial_rel.x);
-
-    for (int i = 0; i < STEPS; i++) {
-        update_simulation(sim);
-    }
-
-    Vec3 final_rel = vec3_sub(earth->global_position, sun->global_position);
-    double theta_final = atan2(final_rel.y, final_rel.x);
-    double delta = theta_final - theta_start;
-
-    INFO("Initial angle: " << theta_start << " rad");
-    INFO("Final angle: " << theta_final << " rad");
-    INFO("Delta: " << delta << " rad");
-
-    REQUIRE_THAT(delta, WithinAbs(0.0172042841, 1e-6));
-    REQUIRE(delta > 0.0);
-}