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)

gdextension/simulation/gdextension/simulation.gdextension

@@ -1,10 +0,0 @@
-[configuration]
-entry_symbol = "gdextension_entry"
-compatibility_minimum = "4.2"
-
-[libraries]
-linux.x86_64 = "res://gdextension/bin/linux/libsimulation.so"
-linux.arm64 = "res://gdextension/bin/linux/libsimulation.so"
-windows.x86_64 = "res://gdextension/bin/windows/libsimulation.dll"
-macos.x86_64 = "res://gdextension/bin/macos/libsimulation.dylib"
-macos.arm64 = "res://gdextension/bin/macos/libsimulation.dylib"