feat(demo): add diamond-square procedural terrain demo

feat(demo): add diamond-square procedural terrain demo

Add DiamondSquareLandscape demo showcasing procedural terrain
generation with 3 colored light sources.

Update all demos to use global LightingManager from ViewPanel instead
of creating per-demo instances. Update Quaternion API calls from
setQuaternion() to set().

Squashed commits:
- feat(demo): add diamond-square procedural terrain demo
- refactor(demos): update demos to use global LightingManager
- refactor: update demos to use Quaternion.set() instead of setQuaternion()