give collectibales physics layer

this keeps keys, dropped-hedz and cash from "colliding" with the camera arm

src/head_drop.rs

@@ -1,6 +1,7 @@
 use crate::{
-    GameState, billboards::Billboard, global_observer, loading_assets::GameAssets, player::Player,
-    sounds::PlaySound, squish_animation::SquishAnimation,
+    GameState, billboards::Billboard, global_observer, loading_assets::GameAssets,
+    physics_layers::GameLayer, player::Player, sounds::PlaySound,
+    squish_animation::SquishAnimation,
 };
 use avian3d::prelude::*;
 use bevy::prelude::*;
@@ -40,6 +41,7 @@ fn on_head_drop(trigger: Trigger<HeadDrops>, mut commands: Commands, assets: Res
             ExternalImpulse::new(spawn_dir * 180.).with_persistence(false),
             LockedAxes::ROTATION_LOCKED,
             RigidBody::Dynamic,
+            CollisionLayers::new(LayerMask(GameLayer::Collectibles.to_bits()), LayerMask::ALL),
             CollisionEventsEnabled,
             Restitution::new(0.6),
         ))