Fix: rejoining stacked players on top of each other (one fell through)

All players spawned at the exact same PlayerSpawn point and were on the
same collision layer, so two overlapping CharacterBody3Ds triggered a
physics depenetration shove that could launch one through the 1-unit-thick
floor.

- server.gd: golden-angle spawn spread -- each join lands ~1m+ from the
  others, growing outward, never exactly repeating.
- server_player.tscn + player.tscn: players on collision layer 2, mask 1
  (world only) -- players no longer collide with each other at all
  (fine for a social game; also removes the shove).
- player_spawner.gd + player.gd: hard safety net -- if y < -10 (any
  tunnel-through), snap back to the spawn instead of falling forever.

Verified: 3 server players spawn 1.15m+ apart, none fall, all rest on the
floor. 2p multiplayer + network_client smokes green.

Client-side -- rebuild the exe.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

scripts/player_spawner.gd

@@ -14,6 +14,10 @@ const GRAVITY := 9.8
 
 var peer_id: int = -1
 
+## Where the server spawned this player (set by server.gd). Used as the
+## respawn point if the player somehow ends up below the world.
+var spawn_position: Vector3 = Vector3(0, 1.5, 0)
+
 ## Highest input seq applied authoritatively so far (echoed in snapshots so
 ## the client can drop acked inputs and replay the rest).
 var last_applied_seq: int = 0
@@ -38,6 +42,13 @@ func _physics_process(delta: float) -> void:
 
 	rotation.y = _look_yaw
 
+	# Safety net: if a player ends up below the world (physics shove, tunnel
+	# through the floor), put them back on their spawn instead of falling
+	# forever.
+	if global_position.y < -10.0:
+		global_position = spawn_position
+		velocity = Vector3.ZERO
+
 
 func _integrate(move_x: float, move_z: float, jump: bool, delta: float) -> void:
 	if not is_on_floor():