fix(app): reduce session spinner CPU usage - #42952

Summary

• replace 25 per-dot CSS opacity animations with one shared pre-rendered APNG timeline
• preserve the eight source poses, smooth ease-out interpolation, currentColor, SVG props, and reduced-motion behavior
• use a 1x animated alpha map plus one vector dot clip so every square keeps the old SVG edges at any display size
• render the baked alpha mask directly and apply runtime color through an SVG alpha filter
• add a deterministic generator for the 108-frame, 1.2-second alpha animation
• add a Storybook stress story that fills the viewport with the shared component

Benchmark

Measured at the production 16 px size in headed Chromium with hardware acceleration enabled on an NVIDIA RTX 3080 Ti Laptop GPU and 20 logical processors. Each result is the median of three five-second samples.

This reduces total CPU by about 76% for the original 20-indicator stress case.

Testing

• bun typecheck in packages/app
• bun run typecheck:e2e in packages/app
• bun typecheck in packages/session-ui
• bun test in packages/session-ui (87 passed)
• deterministic APNG regeneration check
• Chromium pixel check confirms the radar changes direction after 300 ms
• Chromium edge check confirms the Baked Mask has the same hard square boundaries as the old SVG at 32 px
• Chromium checks for reduced motion and currentColor

The full pre-push monorepo type check passed 32 of 33 packages. packages/www is blocked because its generated Blume Astro config imports @astrojs/mdx, which is not linked as a direct workspace dependency. No website files are changed by this PR.