give tile occupancy its own class, BuildingGrid

Eleven methods maintained m_tileOccupancy by hand — place, deconstruct,
removeBuilding, placeImmediate, tickDeconstruction, findRotateInPlaceTarget and
tryDirectCoupleDeposit all indexed a raw std::map<std::pair<int,int>, BuildingId>
directly, so the invariant "occupancy stays in sync with placement" was
re-implemented at every call site. They now ask and tell a small owned index
instead: occupy / release / isOccupied / findOwner.

BuildingGrid is a member of BuildingSystem, not a peer system: it has no
per-tick behaviour and nothing outside BuildingSystem touches it.

The internal keying stays std::pair<int,int> rather than moving to QPoint. The
checksum folds the entries in map iteration order, so the comparator is part of
the determinism contract; changing it is a separate decision, not a side effect
of this move. Verified with a golden-checksum capture before and after — all
four sample ticks identical.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG

src/lib/sim/CMakeLists.txt

@@ -12,6 +12,7 @@ SET(HDRS
     ${CMAKE_CURRENT_SOURCE_DIR}/BeltSystem.h
     ${CMAKE_CURRENT_SOURCE_DIR}/Building.h
     ${CMAKE_CURRENT_SOURCE_DIR}/BuildingConfig.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/BuildingGrid.h
     ${CMAKE_CURRENT_SOURCE_DIR}/BuildingSystem.h
     ${CMAKE_CURRENT_SOURCE_DIR}/EntityHitTest.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayout.h
@@ -36,6 +37,7 @@ SET(SRCS
     ${CMAKE_CURRENT_SOURCE_DIR}/BeltSlot.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BeltSystem.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BuildingConfig.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/BuildingGrid.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BuildingSystem.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/EntityHitTest.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipStatsCalculator.cpp