Add OpticsScene.fromJSON() for scene deserialization Implements a static fromJSON() counterpart to toJSON(), enabling saved scenes to be fully restored. Covers all 19 element types (light sources, mirrors, glass, blockers, gratings). Unknown types are silently skipped for forward-compatibility. Also exports the previously missing ContinuousSpectrumSource and CircleBlocker from the model barrel (index.ts). https://claude.ai/code/session_01P82AYHGaNXToVsaiMCnKEv - #6