Add galaxy parameter UI, star selection, and movement/physics scaffolding

Galaxy creation scene (Bevy 0.16):
- Split into module folder: params.rs, mod.rs (generation + spawn), ui.rs,
  selection.rs
- GalaxyParams resource (seed, count, arms, vertical_arms, size, twist,
  vertical_twist) with generation counter for change detection
- Control panel: 7 +/- slider rows + Regenerate button, no native Bevy slider
  widget so uses label + icon buttons
- Info panel: live-refreshes on selection change via despawn_related::<Children>
- Click-to-select: screen-space picking (project each star to viewport, closest
  within 18px threshold wins); selected star lerps scale to 2.2x
- Generation faithfully ports the docs TS reference including vertical arms
- Regeneration system: despawns GalaxyScene root only, preserves UI panels

Camera:
- Camera2d -> Camera3d + OrbitCamera (left-drag yaw/pitch, scroll zoom,
  pitch/distance clamped to avoid gimbal)
- Orbit drag suppressed when cursor over UI

New plugins (scaffolding):
- movement/: Velocity, MaxSpeed, TurnRate, Drag, MoveTarget, Player components
  + click-to-move (no player spawned yet)
- physics/: pure-data geometry (ray_vs_sphere, overlaps, separate,
  segment_vs_sphere) with 7 passing unit tests; no systems wired yet
- star_map/: plugin skeleton gated on AppState::InGame

Shared:
- ui/util.rs: cursor_over_ui helper for UI-hover detection
- docs: ARCH-9 decision record (custom movement & collision, no physics engine)

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

@@ -0,0 +1,52 @@
+use bevy::prelude::*;
+
+/// Which plane the orbit lies on.
+#[derive(Debug, Clone, Copy, Default)]
+pub enum OrbitPlane {
+    /// XZ plane (Y up). Typical for planets around a star viewed from above.
+    #[default]
+    Horizontal,
+    /// XY plane (Z up). Useful for UI elements or top-down 2D-style orbits.
+    Vertical,
+}
+
+/// An entity that orbits around its parent's origin.
+/// Attach as a child of the orbit center; local Transform is updated each frame.
+#[derive(Component, Debug, Clone, Copy)]
+pub struct Orbit {
+    pub angle: f32,
+    pub angular_velocity: f32,
+    pub radius: f32,
+    pub plane: OrbitPlane,
+}
+
+impl Default for Orbit {
+    fn default() -> Self {
+        Self {
+            angle: 0.0,
+            angular_velocity: 0.5,
+            radius: 10.0,
+            plane: OrbitPlane::Horizontal,
+        }
+    }
+}
+
+/// Advance each Orbit's angle and recompute local translation.
+/// The orbit center is the parent entity (via Bevy's parent-child hierarchy).
+pub fn update_orbits(
+    mut query: Query<(&mut Orbit, &mut Transform)>,
+    time: Res<Time>,
+) {
+    let dt = time.delta_secs();
+    for (mut orbit, mut transform) in &mut query {
+        orbit.angle += orbit.angular_velocity * dt;
+        let (sin, cos) = orbit.angle.sin_cos();
+        let (x, y, z) = match orbit.plane {
+            OrbitPlane::Horizontal => (cos * orbit.radius, 0.0, sin * orbit.radius),
+            OrbitPlane::Vertical => (cos * orbit.radius, sin * orbit.radius, 0.0),
+        };
+        transform.translation.x = x;
+        transform.translation.y = y;
+        transform.translation.z = z;
+    }
+}