Add 3D solar system view with Threlte and velocity tails in 2D

- 3D scene: Threlte/Three.js with starfield, orbit lines, planet meshes,
  sun glow, OrbitControls (orbit/zoom/pan), point lighting
- 2D velocity tails: gradient lines showing planet velocity direction/magnitude
- New WASM API: get_body_velocities_at_epoch(), get_orbit_points()
- Orbit ellipses computed in Rust, rendered in both 2D and 3D views
- Ecliptic-to-Three.js coordinate mapping (Y-up convention)
- View toggle now switches between working 2D and 3D renderers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/orbital-mechanics/src/orbits.rs

@@ -75,6 +75,27 @@ pub fn position_at_epoch(bodies: &[CelestialBody], body_id: usize, jd: f64) -> V
     }
 }
 
+/// Compute velocity of a body via finite differencing (AU/day).
+pub fn velocity_at_epoch(bodies: &[CelestialBody], body_id: usize, jd: f64) -> Vector3<f64> {
+    let dt = 0.01; // days
+    let p1 = position_at_epoch(bodies, body_id, jd - dt);
+    let p2 = position_at_epoch(bodies, body_id, jd + dt);
+    (p2 - p1) / (2.0 * dt)
+}
+
+/// Compute velocities of all bodies at a given epoch.
+/// Returns a flat Vec of [vx, vy, vz, ...] in AU/day.
+pub fn all_velocities_at_epoch(bodies: &[CelestialBody], jd: f64) -> Vec<f64> {
+    let mut result = Vec::with_capacity(bodies.len() * 3);
+    for i in 0..bodies.len() {
+        let vel = velocity_at_epoch(bodies, i, jd);
+        result.push(vel.x);
+        result.push(vel.y);
+        result.push(vel.z);
+    }
+    result
+}
+
 /// Compute positions of all bodies at a given epoch.
 /// Returns a flat Vec of [x, y, z, x, y, z, ...] in AU.
 pub fn all_positions_at_epoch(bodies: &[CelestialBody], jd: f64) -> Vec<f64> {