feat(physics): collision resolution + proximity queries (P2)

Implement the kinematic physics & collision layer per ARCH-7 (no physics
engine; distance-based collision + spatial queries).

physics/
- components.rs (new): BodyMass, ProximitySensor, CollisionEvent, ProximityEvent
- geometry.rs: add sphere_overlap() + split_by_inverse_mass() with unit tests
- systems.rs: collision pipeline (detect_collisions -> resolve_collisions)
  with mass-weighted separation and inbound-velocity cancel, plus
  update_proximity_sensors spatial query and a log_physics_events observer.
  Core math factored into pure plan_collisions() with unit tests.
- mod.rs: PhysicsPlugin registers events, FrameCollisions resource, and the
  InGame systems ordered after kinematic integration.

in_system/scene.rs: give the player ship a BoundingVolume collider, BodyMass,
and a ProximitySensor so it participates in collision and spatial queries.

Bodies with a Velocity are movable; scenery is immovable. 19 unit tests pass;
no new compiler/clippy warnings introduced.

apps/game/src/gameplay/in_system/scene.rs

@@ -13,6 +13,7 @@ use crate::gameplay::galaxy::{
     SystemContents,
 };
 use crate::gameplay::in_system::DockedState;
+use crate::gameplay::physics::{BodyMass, ProximitySensor};
 
 /// Tracks the currently active system for gameplay.
 #[derive(Resource, Debug, Clone, Default)]
@@ -45,6 +46,16 @@ pub struct DockingTarget {
 /// Offset from station where player ship spawns when docked.
 const DOCKED_OFFSET: Vec3 = Vec3::new(1.5, 0.0, 0.0);
 
+/// World-space collision radius for the player ship. Matches its scaled
+/// visual extent (a ~0.12-scale cuboid) so it cannot overlap stations,
+/// planets, or the star.
+const PLAYER_COLLIDER_RADIUS: f32 = 0.15;
+
+/// World-space radius of the player ship's proximity sensor — the range at
+/// which nearby bodies (stations, belts, the star) register as in-range via
+/// `ProximityEvent`s.
+const PLAYER_SCAN_RADIUS: f32 = 8.0;
+
 /// Represents a docking target (either a station or a habitable planet).
 #[derive(Debug, Clone)]
 struct DockingTargetInfo {
@@ -352,6 +363,15 @@ fn spawn_player_ship_docked(
             Docked {
                 station_entity: Entity::PLACEHOLDER, // Will be set when we find the actual station
             },
+            // Physical presence: a collider for the collision pipeline and a
+            // proximity sensor for spatial queries. `BodyMass` weights
+            // ship-to-ship separation; the ship has no `Velocity` while docked
+            // so the resolver treats it as immovable until undock.
+            BoundingVolume {
+                radius: PLAYER_COLLIDER_RADIUS,
+            },
+            BodyMass(1.0),
+            ProximitySensor::new(PLAYER_SCAN_RADIUS),
             Transform::from_translation(ship_position).with_scale(Vec3::splat(0.12)),
             Visibility::default(),
             InheritedVisibility::default(),