refactor(game): extract EntityController from GameHandler (step 1.3)

Moves entity lifecycle, name/creature/game-object caches, transport GUID
tracking, and the entire update-object pipeline out of GameHandler into a
new EntityController class (friend-class pattern, same as CombatHandler
et al.).

What moved:
- applyUpdateObjectBlock() — 1,520-line core of all entity creation,
  field updates, and movement application
- processOutOfRangeObjects() / finalizeUpdateObjectBatch()
- handleUpdateObject() / handleCompressedUpdateObject() / handleDestroyObject()
- handleNameQueryResponse() / handleCreatureQueryResponse()
- handleGameObjectQueryResponse() / handleGameObjectPageText()
- handlePageTextQueryResponse()
- enqueueUpdateObjectWork() / processPendingUpdateObjectWork()
- playerNameCache, playerClassRaceCache_, pendingNameQueries
- creatureInfoCache, pendingCreatureQueries
- gameObjectInfoCache_, pendingGameObjectQueries_
- transportGuids_, serverUpdatedTransportGuids_
- EntityManager (accessed by other handlers via getEntityManager())

8 opcodes re-registered by EntityController::registerOpcodes():
  SMSG_UPDATE_OBJECT, SMSG_COMPRESSED_UPDATE_OBJECT, SMSG_DESTROY_OBJECT,
  SMSG_NAME_QUERY_RESPONSE, SMSG_CREATURE_QUERY_RESPONSE,
  SMSG_GAMEOBJECT_QUERY_RESPONSE, SMSG_GAMEOBJECT_PAGETEXT,
  SMSG_PAGE_TEXT_QUERY_RESPONSE

Other handler files (combat, movement, social, spell, inventory, quest,
chat) updated to access EntityManager via getEntityManager() and the
name cache via getPlayerNameCache() — no logic changes.

Also included:
- .clang-tidy: add modernize-use-nodiscard,
  modernize-use-designated-initializers; set -std=c++20 in ExtraArgs
- test.sh: prepend clang's own resource include dir before GCC's to
  silence xmmintrin.h / ia32intrin.h conflicts during clang-tidy runs

Line counts:
  entity_controller.hpp  147 lines  (new)
  entity_controller.cpp  2172 lines (new)
  game_handler.cpp       8095 lines (was 10143, −2048)

Build: 0 errors, 0 warnings.

.clang-tidy

@@ -16,6 +16,8 @@ Checks: >
   modernize-deprecated-headers, 
   modernize-make-unique, 
   modernize-make-shared, 
+  modernize-use-nodiscard, 
+  modernize-use-designated-initializers, 
   readability-braces-around-statements, 
   readability-container-size-empty, 
   readability-delete-null-pointer, 
@@ -35,6 +37,7 @@ WarningsAsErrors: ''
 # Suppress the noise from GCC-only LTO flags in compile_commands.json.
 # clang doesn't support -fno-fat-lto-objects; this silences the harmless warning.
 ExtraArgs:
+  - -std=c++20
   - -Wno-ignored-optimization-argument
 
 HeaderFilterRegex: '^.*/include/.*\.hpp$'