sphere-sim — interactive harness

One WebGL2 context, five viewports: the room, and the four projector rasters as quadrants of the one framebuffer SOS actually drives (PARAMETERS.md §3.4). Sliders carry the provenance class of the constant they move; anything marked ASSUME is a number nobody has measured. Every photometric metric is PROVISIONAL. The GPU↔CPU parity delta is at the top of the metrics panel and is never hidden.