PC3 Phase 2.3: networked play path in the game

- Session autoload: carries {world_id, host, port} from the world browser
  to the game scene.
- world_list.gd "Enter World": POST /api/worlds/{id}/join -> Session.begin
  -> load game.tscn. On 503 (backend has no orchestration) falls back to
  local single-player; other errors surface on the card.
- api_client.gd: join_world().
- game.gd (new, on game.tscn root): local single-player by default; when
  Session.active, connects a NetworkClient to the world's dedicated server
  and runs networked -- forwards the local Player's input each physics
  frame, snaps to the server position past a generous drift threshold
  (the Player keeps predicting locally with real collision), renders other
  players from snapshots (labelled capsules, lerped -- avatar appearance
  sync is a follow-up, the server doesn't replicate loadouts yet), and
  returns to the world list on disconnect.
- player.gd: exposes net_input (the input frame it applied) and
  apply_server_correction().
- network_client.gd: send_input() + get_own_server_state() for callers that
  do their own collision-aware prediction.
- scripts/mp_game_flow_client.gd + run_game_multiplayer_test.sh: two real
  clients each register -> join via the API -> game.tscn -> networked;
  A sees B and moves. PASS.

Verified end to end against an orchestration-enabled backend. Non-networked
smokes green (enter_world 9/9, world_list 5/5, player 15/15,
avatar_in_world 10/10, main_menu_full_flow 12/12).

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

scripts/run_game_multiplayer_test.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+# Full networked game flow, two real client processes:
+#   register -> POST /api/worlds/{id}/join -> Session -> game.tscn -> networked.
+# Client A must see client B rendered as a remote player and its own movement
+# must round-trip. Requires a backend with GAME_SERVERS_ENABLED and the Godot
+# binary reachable from it.
+#
+# Usage: scripts/run_game_multiplayer_test.sh <backend_base_url> <world_id>
+set -uo pipefail
+cd "$(dirname "$0")/.."
+
+BASE_URL="${1:?usage: run_game_multiplayer_test.sh <backend_url> <world_id>}"
+WORLD_ID="${2:?usage: run_game_multiplayer_test.sh <backend_url> <world_id>}"
+GODOT_BIN="${GODOT_BIN:-godot}"
+command -v "$GODOT_BIN" >/dev/null 2>&1 || GODOT_BIN="$HOME/godot/godot"
+
+export AGP_API_BASE_URL="$BASE_URL"
+
+cleanup() { kill "${B_PID:-}" 2>/dev/null; wait 2>/dev/null; }
+trap cleanup EXIT
+
+echo "=== client B: join world $WORLD_ID and hold ==="
+"$GODOT_BIN" --headless --path . --script res://scripts/mp_game_flow_client.gd -- \
+	--role b --world "$WORLD_ID" >/tmp/mpflow_b.log 2>&1 &
+B_PID=$!
+for _ in $(seq 1 40); do grep -q 'RESULT b in_world' /tmp/mpflow_b.log && break; sleep 1; done
+grep -q 'RESULT b in_world' /tmp/mpflow_b.log || { echo "B never entered the world:"; tail -5 /tmp/mpflow_b.log; echo "=== SMOKE RESULT: FAIL ==="; exit 1; }
+
+echo "=== client A: join world $WORLD_ID, move, look for B ==="
+"$GODOT_BIN" --headless --path . --script res://scripts/mp_game_flow_client.gd -- \
+	--role a --world "$WORLD_ID" 2>&1 | tee /tmp/mpflow_a.log
+A_LINE=$(grep '^RESULT a' /tmp/mpflow_a.log || true)
+
+echo
+echo "A: ${A_LINE:-<none>}"
+echo "B: $(grep '^RESULT b' /tmp/mpflow_b.log || echo '<none>')"
+
+if echo "$A_LINE" | grep -q 'moved=true' && echo "$A_LINE" | grep -qE 'remotes=[1-9]'; then
+	echo "=== SMOKE RESULT: PASS ==="
+	exit 0
+fi
+echo "=== SMOKE RESULT: FAIL ==="
+exit 1