Schedule VR shadow-map updates

[reality2-roycdavies]

Summary

• schedule sun-shadow map redraws only in WebXR, leaving desktop rendering unchanged
• redraw at 24 Hz while moving and 8 Hz while stationary, with immediate invalidation for terrain/chunk changes
• add focused scheduler tests and bump the browser cache version to VR Forest v18

Validation

• npm test — 215 tests passed
• node --check on the entry point, scheduler, and scheduler tests
• git diff --check
• headless production-style browser load fetched the complete module graph, including main.js?v=205 and shadow-scheduler.js?v=205; WebGL2 rendering remains unavailable in this headless environment

Headset quality checkpoint

After deployment, verify that tree and ground shadows do not visibly step or lag while walking, weather/time lighting remains smooth while stationary, and shadows appear immediately as new terrain chunks load.