Slow orbital periods for readability at galaxy-inspection scale

- Introduce ORBITAL_PERIOD_SCALE = 4.0 multiplier so planets/stations/
  gas clouds/anomalies all orbit visibly slower. At inspection-scale
  zoom the un-scaled periods (≈2.5s per unit of radius) made bodies
  spin like tops; tuned for a calm browsing pace.
- Default Orbit angular_velocity 0.5 → 0.15 (≈40s per revolution).
- Update orbital_period unit tests for the new scale factor.

apps/game/src/gameplay/movement/orbit.rs

@@ -24,7 +24,9 @@ impl Default for Orbit {
     fn default() -> Self {
         Self {
             angle: 0.0,
-            angular_velocity: 0.5,
+            // ~1 full revolution every 40 seconds — calm enough to read at
+            // inspection-scale zoom levels. Callers can override per-instance.
+            angular_velocity: 0.15,
             radius: 10.0,
             plane: OrbitPlane::Horizontal,
         }