Fix: players fall through the floor in multiplayer

scenes/server_player.tscn was a bare CharacterBody3D with NO CollisionShape3D,
so on the server move_and_slide() never hit the ground and is_on_floor() was
always false -> the server player fell forever, and the client (which snaps to
the server's authoritative position past a drift threshold) fell with it.

Give the server player the same CapsuleShape3D (r=0.35, h=1.6) as the client.
The 2-player test now also asserts the server keeps the player on the ground
(on_floor / min_y), which is how this slipped through -- the tests only
checked "position changes" and "sees the other player".

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

scenes/server_player.tscn

@@ -1,6 +1,13 @@
-[gd_scene load_steps=2 format=3]
+[gd_scene load_steps=3 format=3]
 
 [ext_resource type="Script" path="res://scripts/player_spawner.gd" id="1"]
 
+[sub_resource type="CapsuleShape3D" id="CapsuleShape_server_player"]
+radius = 0.35
+height = 1.6
+
 [node name="Player" type="CharacterBody3D"]
 script = ExtResource("1")
+
+[node name="CollisionShape3D" type="CollisionShape3D" parent="."]
+shape = SubResource("CapsuleShape_server_player")