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.gd

@@ -26,6 +26,7 @@ const MOUSE_SENSITIVITY: float = 0.003
 
 var _input_direction := Vector2.ZERO
 var _camera_pitch: float = -0.12
+var _spawn_position: Vector3 = Vector3(0, 1.5, 0)
 
 ## Vertical offset applied to the assembled avatar so its feet (origin y=0)
 ## line up with the bottom of the player's collision capsule.
@@ -69,6 +70,7 @@ func _ready() -> void:
 	else:
 		push_warning("player.gd: no PlayerSpawn marker found; spawning at origin")
 		global_position = Vector3(0, 1.0, 0)
+	_spawn_position = global_position
 
 	# The spring arm must not collide with the player's own capsule (they're
 	# on the same layer), or the camera gets yanked right up against the
@@ -192,6 +194,11 @@ func _physics_process(delta: float) -> void:
 
 	move_and_slide()
 
+	# Safety net -- never fall forever if something goes wrong.
+	if global_position.y < -10.0:
+		global_position = _spawn_position
+		velocity = Vector3.ZERO
+
 	net_input = {
 		"move_x": direction.x, "move_z": direction.z,
 		"jump": jumped, "look_yaw": rotation.y,