[codex] Improve runtime efficiency on mobile and desktop

[Metadrew]

What changed

This PR reduces runtime cost across both mobile and desktop without changing the public API.

• parks the render loop completely when nothing is dirty instead of continuously scheduling frames
• adds adaptive mobile quality scaling by capping effective DPR and downsampling blur passes on constrained / coarse-pointer devices
• adds per-frame desktop caches for bounds, padded rects, dynamic/media descendant lookups, media fit styles, and canvas contexts to avoid repeated CPU work across overlapping glass panels
• updates the demo page to use the local built bundle and refreshes the README / demo limitations text to reflect the new runtime behavior

Why

The original implementation looked great but could be CPU intensive on mobile and still repeated a lot of layout/query work on desktop when multiple glass panels overlapped. This pass keeps the visual result intact while reducing idle work and trimming the hottest repeated paths.

Impact

• static scenes now go fully idle until something changes
• mobile devices should run cooler and spend less time in blur / raster work
• desktop keeps full quality while doing less repeated per-frame CPU bookkeeping
• demo and documentation now describe the optimized behavior more accurately

Validation

• npm ci
• npm run build