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

scenes/server_player.tscn

@@ -7,6 +7,8 @@ radius = 0.35
 height = 1.6
 
 [node name="Player" type="CharacterBody3D"]
+collision_layer = 2
+collision_mask = 1
 script = ExtResource("1")
 
 [node name="CollisionShape3D" type="CollisionShape3D" parent="."]