Add implementation plan for OpenGL 3D view

Phased TDD plan: pure math/data core (Mat4, OrbitCamera, ShadeColor,
PickingCodec, MeshAccumulator, IndexSetBuilder, SceneBuilder), GL
infrastructure (shaders, GlScene, PickingFramebuffer, HighlightOverlay,
GLViewport), GUI wiring, manual verification, and packaging.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>