fix(camera): prove the boom shortens, and record how to repeat it

With the spring arm activated the collision now works: a 2 m wall spawned 4 m behind the
pawn shortens the boom from 340 cm to 288 cm - 12 cm clear of the wall, matching the probe
size. -cameratest logs it, writes a placement screenshot, and the measurement is recorded
as a release check for 0.2.3.

Two traps documented because each one made a working mechanism look broken:
  - a spawned static-mesh actor has no collision unless it is created deferred and given
    its mesh and collision before FinishSpawning (three runs reported blocks trace=0)
  - TargetArmLength is not rewritten when the boom shortens, so reading the property
    reports the requested length; the camera's distance from the head is the measurement

Writes the 0.2.3 changelog entry and prunes the section from TODO.md.

Game/GigLine/Source/GigLine/GigLineCharacter.h

@@ -208,7 +208,7 @@ protected:
 	void UpdateCameraCollisionTest(float DeltaSeconds);
 
 	UPROPERTY()
-	TObjectPtr<class ABlockingVolume> CameraTestWall;
+	TObjectPtr<class AStaticMeshActor> CameraTestWall;
 
 	FVector LastWallNormal = FVector::ZeroVector;
 	bool bCameraTest = false;