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/mod.rs

@@ -0,0 +1,26 @@
+use bevy::prelude::*;
+
+pub mod components;
+pub mod input;
+pub mod kinematic;
+pub mod orbit;
+
+pub struct MovementPlugin;
+
+impl Plugin for MovementPlugin {
+    fn build(&self, app: &mut App) {
+        // No state gating for slice 1: systems are no-ops when no Player exists.
+        // When AppState::InGame is wired up, gate these on `in_state(AppState::InGame)`
+        // and consider moving them to FixedUpdate for SpacetimeDB determinism.
+        app.add_systems(
+            Update,
+            (
+                input::click_to_move,
+                kinematic::steer_to_target,
+                kinematic::integrate_velocity,
+                orbit::update_orbits,
+            )
+                .chain(),
+        );
+    }
+}