Use std::optional instead of sentinel values for absent data

Replace sentinel values that signalled "no value" with std::optional
across simulation, UI, and balancing code:

- BuildingId references: DeliverScrapBehavior::deliveryBay,
  Simulation::m_hqBuildingId, the return types of BuildingSystem::place
  and Simulation::tryPlaceBuilding (previously kInvalidBuildingId on
  failure), GameWorldView building/site-at-tile lookups and demolish
  hover, SelectedBuildingPanel::m_singleBuildingId.
- Command id fields: the five id-carrying commands now hold
  optional<BuildingId>; CommandSerializer and Simulation::apply updated.
  The serialized replay format is unchanged.
- Index sentinels: BlueprintPanel::m_activeIndex,
  BalancingWindow::m_inspectedArenaIndex, ArenaSimulation winnerTeam,
  and the moduleIndex grid cells in ShipLayoutDialog/ShipLayoutPreview.
  ShipLayoutDialog::m_activeModuleIndex was a tri-state, so it is
  modelled as bool m_removeMode + optional<int>.

Building/ConstructionSite id defaults keep kInvalidBuildingId (identity,
not an absent reference); entt::null and config -1 domain values are
left as-is.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

src/lib/ecs/system/ai/DeliverScrapEvaluator.cpp

@@ -31,7 +31,7 @@ void DeliverScrapEvaluator::evaluate(EntityAdmin& admin, const BuildingSystem& b
             }
 
             // Assign nearest SalvageBay if not yet assigned.
-            if (deliver.deliveryBay == kInvalidBuildingId)
+            if (!deliver.deliveryBay.has_value())
             {
                 const Building* bay =
                     buildings.findNearestBuilding(pos.value, BuildingType::SalvageBay);