Phase 7: test server deployment + Windows export

- Fix: export_presets.cfg — platform='Windows Desktop' (correct name)
- Fix: server_main.gd — formatting bug on lag-comp string
- Fix: game_server.gd — auto-detect GDExtension, fall back to GDScript stub
- Add: server/scripts/simulation_server_stub.gd — lightweight SimulationServer in GDScript
- Add: docs/playtest-guide.md — connection instructions, control bindings, server commands
- Add: systemd user service — tactical-shooter-server (enabled, running)
- Add: server config at ~/tactical-shooter-server/server_config.cfg
- Build: Windows 109MB PE32+ client (build/tactical-shooter-windows-x86_64/)
- Build: Linux server binary (78MB) — running on oplabs:34197
- Temporarily disabled GDExtension (needs C++ build fix in entity.cpp enum casting)

server/scripts/game_server.gd

@@ -48,8 +48,11 @@ var entity_to_peer: Dictionary = {}  # entity_id (int) → peer_id (int)
 # ---------------------------------------------------------------------------
 
 func _ready() -> void:
-	# Create the SimulationServer
-	simulation_server = SimulationServer.new()
+	# Create the SimulationServer (stub if GDExtension not compiled)
+	var SimServerClass = load("res://server/scripts/simulation_server_stub.gd")
+	if ClassDB.class_exists(&"SimulationServer"):
+		SimServerClass = ClassDB.instantiate(&"SimulationServer").get_script()
+	simulation_server = SimServerClass.new()
 	simulation_server.set_tick_rate(128)
 
 	# Apply movement config from ServerConfig singleton if available