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/server.gd

@@ -30,6 +30,9 @@ var status_file_path: String = ""
 var players: Dictionary = {}
 var tick: int = 0
 var is_running: bool = false
+## Monotonic count of joins ever -- drives spawn spread so two players never
+## land on the same spot (which used to shove one through the floor).
+var _join_count: int = 0
 
 var _player_scene: PackedScene = preload("res://scenes/server_player.tscn")
 var _level_scene: PackedScene = preload("res://scenes/worlds/test_level.tscn")
@@ -65,6 +68,16 @@ func _ready() -> void:
 	print("[Server] world=%s listening on ENet %d (%d Hz)" % [world_id, port, PHYSICS_HZ])
 
 
+## A spawn point offset from the level's marker so joins don't stack. Golden-
+## angle spiral: evenly spread, never exactly repeats, grows slowly outward.
+func _spawn_for(index: int) -> Vector3:
+	if index == 0:
+		return _spawn_point
+	var angle := index * 2.399963  # golden angle (radians)
+	var radius := 0.9 + 0.25 * float(index)
+	return _spawn_point + Vector3(cos(angle) * radius, 0.0, sin(angle) * radius)
+
+
 func _parse_args() -> void:
 	var args := OS.get_cmdline_user_args()
 	var i := 0
@@ -127,7 +140,10 @@ func _on_peer_connected(peer_id: int) -> void:
 	player.name = "Player_%d" % peer_id
 	player.peer_id = peer_id
 	add_child(player)
-	player.global_position = _spawn_point
+	var spawn := _spawn_for(_join_count)
+	_join_count += 1
+	player.global_position = spawn
+	player.spawn_position = spawn
 	players[peer_id] = player
 	print("[Server] peer %d joined (%d players)" % [peer_id, players.size()])
 	_write_status()