Remove redundant calculate_true_anomaly() function

It was a duplicate of the true_anomaly calculation already performed
by cartesian_to_orbital_elements(). Added a note in old_tests for
future refactor suggesting the replacement usage.

old_tests/test_periapsis_burn.cpp

@@ -8,6 +8,17 @@
 #include "../src/orbital_mechanics.h"
 #include <cmath>
 
+// NOTE: calculate_true_anomaly() was removed from the public API.
+// It was redundant with cartesian_to_orbital_elements() which already
+// computes true_anomaly as part of OrbitalElements.
+//
+// Suggested replacement for later refactor:
+//   OrbitalElements orbit = cartesian_to_orbital_elements(r, v, parent->mass);
+//   double true_anomaly = orbit.true_anomaly;
+//
+// Or even simpler — since the sim already calls cartesian_to_orbital_elements()
+// each frame, just read craft->orbit.true_anomaly directly.
+
 // Test prograde burn at periapsis (true anomaly = 0)
 // Verifies that the maneuver executes correctly when starting at periapsis
 TEST_CASE("Prograde burn at periapsis preserves periapsis distance", "[maneuver][periapsis]") {

src/orbital_mechanics.cpp

@@ -458,52 +458,3 @@ Vec3 calculate_eccentricity_vector(Vec3 r, Vec3 v, Vec3 h, double mu) {
     Vec3 r_over_mag = vec3_scale(r, 1.0 / r_mag);
     return vec3_sub(v_cross_h_over_mu, r_over_mag);
 }
-
-// Calculate true anomaly from position and velocity vectors
-double calculate_true_anomaly(Vec3 r, Vec3 v, Vec3 e_vec, double e_mag, double r_mag) {
-    // For near-circular orbits, eccentricity vector is near-zero
-    // Compute true anomaly as the angle in the orbital plane
-    if (e_mag < 1e-10) {
-        Vec3 h = vec3_cross(r, v);
-        double h_mag = vec3_magnitude(h);
-        if (h_mag < 1e-10) return 0.0;
-
-        // Create a coordinate system in the orbital plane
-        Vec3 z_hat = vec3_scale(h, 1.0 / h_mag);
-        // Choose x-axis as cross product of Z (world up) and orbit normal
-        // This gives a consistent reference direction in the orbital plane
-        Vec3 world_z = {0.0, 0.0, 1.0};
-        Vec3 x_hat = vec3_cross(world_z, z_hat);
-        double x_hat_mag = vec3_magnitude(x_hat);
-        if (x_hat_mag < 1e-10) {
-            // Orbit is equatorial, use world X as reference
-            x_hat = (Vec3){1.0, 0.0, 0.0};
-        } else {
-            x_hat = vec3_scale(x_hat, 1.0 / x_hat_mag);
-        }
-        Vec3 y_hat = vec3_cross(z_hat, x_hat);
-
-        // Project position onto this orbital plane coordinate system
-        double x_proj = vec3_dot(r, x_hat);
-        double y_proj = vec3_dot(r, y_hat);
-
-        // True anomaly is the angle in the orbital plane
-        double nu = atan2(y_proj, x_proj);
-        if (nu < 0) nu += 2.0 * M_PI;
-        return nu;
-    }
-
-    // Standard calculation using eccentricity vector
-    double cos_nu = vec3_dot(e_vec, r) / (e_mag * r_mag);
-    cos_nu = fmax(-1.0, fmin(1.0, cos_nu));
-    double nu = acos(cos_nu);
-
-    // Determine correct quadrant using cross product
-    Vec3 r_cross_v = vec3_cross(r, v);
-    double r_cross_v_dot_e = vec3_dot(r_cross_v, e_vec);
-    if (r_cross_v_dot_e < 0) {
-        nu = 2.0 * M_PI - nu;
-    }
-
-    return nu;
-}

src/orbital_mechanics.h

@@ -56,7 +56,4 @@ double angular_distance(double a, double b);
 // Calculate eccentricity vector from state vectors
 Vec3 calculate_eccentricity_vector(Vec3 r, Vec3 v, Vec3 h, double mu);
 
-// Calculate true anomaly from position and velocity vectors
-double calculate_true_anomaly(Vec3 r, Vec3 v, Vec3 e_vec, double e_mag, double r_mag);
-
 #endif