P7.9: fix headless export — resolve Resource class_name ordering + duplicate RoundManager + simulation stub API

- Fix dead code in server_main.gd (unreachable lag compensation print)
- Remove WeaponData type hints from weapon_server.gd (Resource class_name in Node scripts = headless fail)
- Use preload() instead of WeaponData class_name in weapon_definitions.gd for const refs
- Lazy-init WEAPONS dict (const can't reference preload members)
- Remove duplicate class_name RoundManager from server/scripts/round/round_manager.gd
- Remove DamageProcessor type hints from round/round_manager.gd and bomb_objective.gd
- Add start()/stop()/can_tick()/tick()/spawn_entity()/despawn_entity() to simulation_server_stub.gd
- Fix get_world_3d() → get_tree().root.world_3d in weapon_server.gd Node context
- Fix var data := → var data = for untyped WeaponDefinitions.get_weapon() returns
- Clean export cache and verify server starts with zero parse errors

server/scripts/economy/buy_menu_handler.gd

@@ -117,7 +117,7 @@ func buy_request(weapon_id: String) -> void:
 ## Process a buy request (also callable directly from server-side code).
 func _process_buy_request(player_id: int, weapon_id: String) -> void:
     # ── 1. Validate weapon exists ──
-    var weapon_data: WeaponData = WeaponDefinitions.get_weapon(weapon_id)
+    var weapon_data = WeaponDefinitions.get_weapon(weapon_id)  # WeaponData — untyped for headless
     if weapon_data == null:
         purchase_denied.emit(player_id, weapon_id, "unknown_weapon")
         _send_buy_denied(player_id, "Unknown weapon: \"%s\"" % weapon_id)
@@ -194,7 +194,7 @@ func get_affordable_weapons(player_id: int) -> Array[Dictionary]:
     for wid in EconomyManager.WEAPON_COSTS.keys():
         var cost: int = EconomyManager.WEAPON_COSTS[wid]
         if money >= cost:
-            var data: WeaponData = WeaponDefinitions.get_weapon(wid)
+            var data = WeaponDefinitions.get_weapon(wid)  # WeaponData — untyped for headless
             affordable.append({
                 "weapon_id": wid,
                 "display_name": data.display_name if data else wid,