0.8.13

A schema for <tosi-b3d> itself, and an emulated headset in the repo.

Added

• sceneSchemas.b3d / b3dSchema(): the host's scene-level attributes, with ranges, units and useful bands: clearColor, glowLayerIntensity, ssao, ssaoStrength, ssaoRadius, timeScale, windSpeed, windBearingDeg, windGust. The attributes that configure a viewer rather than a scene (camera limits, quality, pixel ratio, gamepad and panel chrome, XR and pause behaviour) are declined by name in SCENE_OMITTED.b3d. The drift test covers it like every other schema. ssao marks always in x-deprecated-values. Asked by tosijs-3d-ensemble (#101).
• bun run xr <path> (bin/xr-emulate.ts, repo tooling, not shipped): an emulated headset. Headless Chrome with a fake WebXR device enters VR on a page, screenshots both eyes and drives controllers from script. It is how 0.8.12's headset fixes were found.