Port FindTransitionalPosition, TransitionalInsert, FindEnvCollisions,
AdjustOffset, DoStepDown, ValidateTransition from transition_pseudocode.md.
Outdoor terrain collision with step-down ground contact. Indoor BSP and
object collision deferred to subsequent tasks.
Also adds PhysicsEngine.SampleTerrainZ() which dispatches the terrain Z
query to the right registered landblock by world-space XY position.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
