refactor(demos): adopt new transform API and simplify initialization

refactor(demos): adopt new transform API and simplify initialization

Update all demos to use the 6-parameter Transform.fromAngles and set
methods that accept full Euler rotation (x, y, z, yaw, pitch, roll).
Remove explicit render thread start calls since ViewPanel now auto-starts
on component events. Add initial repaint signals where needed.

OctreeDemo now adds lights to both raytracer and rasterizer lighting
systems for consistent illumination. Rename DiamondSquareLandscape to
TerrainDemo for clarity. Increase voxel opacity in fractal from 100 to
200 for better visibility.