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/mass-driver-wasm/src/api.rs

@@ -46,6 +46,15 @@ pub fn get_body_radii() -> Vec<f64> {
     all.iter().map(|b| b.radius_km).collect()
 }
 
+/// Get velocities of all celestial bodies at a given Julian Date.
+///
+/// Returns a Float64Array of [vx0, vy0, vz0, vx1, vy1, vz1, ...] in AU/day.
+#[wasm_bindgen]
+pub fn get_body_velocities_at_epoch(jd: f64) -> Vec<f64> {
+    let all = bodies::all_bodies();
+    orbits::all_velocities_at_epoch(&all, jd)
+}
+
 /// Get orbit points for a given body, sampled around its full orbit.
 /// Returns a Float64Array of [x0, y0, z0, x1, y1, z1, ...] in AU.
 /// `samples` is the number of points around the orbit.