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)

scripts/network/server_main.gd

@@ -75,7 +75,8 @@ func _ready() -> void:
 	print("[ServerMain]   Maps:       %s" % ServerConfig.map_list)
 	print("[ServerMain]   Headless:   %s" % (DisplayServer.get_name() == &"headless"))
 	print("[ServerMain]   Spawn pts:  Team A=%d, Team B=%d" % [spawn_points_a.size(), spawn_points_b.size()])
-	print("[ServerMain]   Lag comp:   Enabled (64-tick history = %dms)" % [64.0 * 1000.0 / ServerConfig.tick_rate])
+	var lag_comp_ms: float = 64.0 * 1000.0 / ServerConfig.tick_rate
+	print("[ServerMain]   Lag comp:   Enabled (64-tick history = %.1fms)" % lag_comp_ms)
 
 func _exit_tree() -> void:
 	NetworkManager.stop()