add orbital mechanics module documentation to technical reference

docs/technical_reference.md

@@ -111,6 +111,7 @@ struct OrbitalElements {
     double argument_of_periapsis;
 };
 ```
+**Note:** 3D orientation using `inclination`, `longitude_of_ascending_node`, and `argument_of_periapsis` is defined but not yet applied to orbital calculations (deferred implementation).
 
 ### Spacecraft (spacecraft.h)
 ```cpp
@@ -195,6 +196,21 @@ Vector math and gravity calculations. RK4 (Runge-Kutta 4th order) integration wi
 - `rk4_step(Vec3* position, Vec3* velocity, double dt, double body_mass, double parent_mass)` - RK4 integration using position/velocity pointers
 - `evaluate_acceleration(Vec3 relative_pos, double body_mass, double parent_mass)` - computes gravitational acceleration from parent
 
+### Orbital Mechanics (orbital_mechanics.cpp/h)
+Keplerian orbital elements to Cartesian coordinate conversion. Supports all orbit types with planar orbits (3D orientation deferred).
+
+**Key functions:**
+- `orbital_elements_to_cartesian(OrbitalElements elements, double parent_mass, Vec3* out_position, Vec3* out_velocity)` - converts Keplerian elements to local position/velocity
+
+**Implementation:**
+- Circular orbits (e=0): Position on circle, velocity from vis-viva equation
+- Elliptical orbits (0<e<1): Position from polar equation, velocity from vis-viva
+- Parabolic orbits (e≈1): Uses `semi_latus_rectum` (p), position `r = p/(1+cos(ν))`, velocity `v = √(2μ/r)`
+- Hyperbolic orbits (e>1): Same as elliptical with negative semi_major_axis
+- All elements use SI units (meters, radians)
+- `true_anomaly = 0` is at periapsis (closest approach)
+- 3D orientation (inclination, RAAN, argument of periapsis) defined but not yet applied (deferred)
+
 ### Simulation (simulation.cpp/h)
 Simulation state management and updates. SOI detection using Hill sphere: `r_soi = a * (m/M)^(2/5)`.
 
@@ -299,6 +315,7 @@ orbit = {
 ```
 - Uses `orbit` table for orbital elements (same as bodies)
 - Altitude can be used instead of `semi_major_axis` for non-parabolic orbits (convenience feature, added to parent radius)
+- Optional 3D orbital elements (inclination, longitude_of_ascending_node, argument_of_periapsis) are defined but not yet applied (deferred)
 
 **Config format (TOML) - Maneuvers:**
 ```toml