Fix headless class_name dependencies

- Replaced TeamData.Team type hints with int in all scripts
- Added explicit preloads for headless mode class resolution
- Created stub LagCompensation and DamageProcessor scripts
- Fixed PluginManager, SpawnManager, EconomyManager, BuyZone,
  RoundManager, BuyMenuHandler, BombObjective class_name references
- Updated server config to match ServerConfig.gd format

Gray screen root cause: server scripts failed to parse in headless
mode due to Godot 4 class_name loading order (Resource after Node),
leaving server_main.gd non-functional — accepted connections but
never spawned players.

server/scripts/plugin_api/plugin_manifest.gd

@@ -15,7 +15,7 @@
 ##   enabled = true
 
 extends Resource
-class_name PluginManifest
+# class_name PluginManifest — commented out for headless compat
 
 # ---------------------------------------------------------------------------
 # Exported fields