test(objects): add load-time clamp + uniqueId round-trip tests

- ObjectPlacer load: scale=0 clamped to 1.0, missing scale field
  defaults to 1.0 (locks in the load-side guard at line 346).
- ObjectPlacer save→load: uniqueId values survive a full round
  trip (locks in the explicit uniqueId preservation behavior the
  ADT round-trip relies on).

Adds object_placer.cpp to test_editor_units sources.

tests/CMakeLists.txt

@@ -434,6 +434,7 @@ add_executable(test_editor_units
     ${CMAKE_SOURCE_DIR}/tools/editor/quest_editor.cpp
     ${CMAKE_SOURCE_DIR}/tools/editor/content_pack.cpp
     ${CMAKE_SOURCE_DIR}/tools/editor/editor_brush.cpp
+    ${CMAKE_SOURCE_DIR}/tools/editor/object_placer.cpp
     ${CMAKE_SOURCE_DIR}/src/core/logger.cpp
 )
 target_include_directories(test_editor_units PRIVATE