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

scripts/run_multiplayer_2p_test.sh

@@ -37,7 +37,9 @@ echo "server: $(grep -c 'joined' /tmp/mp2p_server.log) joins seen"
 echo "A: ${A_LINE:-<none>}"
 echo "B: ${B_LINE:-<none>}"
 
-if echo "$A_LINE" | grep -q 'moved=true' && echo "$A_LINE" | grep -qE 'remotes=[1-9]'; then
+if echo "$A_LINE" | grep -q 'moved=true' \
+	&& echo "$A_LINE" | grep -qE 'remotes=[1-9]' \
+	&& echo "$A_LINE" | grep -q 'on_floor=true'; then
 	echo "=== SMOKE RESULT: PASS ==="
 	exit 0
 fi