feat(cityflow): add retained Cityflow adapter

[alowpoly]

Summary

• add a source-backed XScreenSaver Cityflow adapter using one retained PolyCSS Morph graph
• prepare the seeded source simulation into canonical transform and color-index tables
• apply sparse prepared state with the merged DOMFORMAT polycss-playback@0 timer/rAF elapsed-catch-up precedent
• keep the runtime at 200 retained box roots / 600 solid quad leaves with no Canvas, SVG scene, runtime rasterization, topology rebuilding, or DOM growth
• add deterministic preparation, pinned native state, browser smoke, frame-sequence visual oracle, and source-only contracts

Why

The initial 800-root CSS-keyframe implementation was GPU/compositor bound and caused per-leaf paint pressure. The source supports an authored count control and scales boxes as 1.8 / sqrt(count), so the product uses 200 boxes to preserve field coverage while reaching the source's 20 ms cadence. Per-box keyframes were replaced by prepared sparse playback rather than changing the renderer.

Validation

• pnpm verify:source-only
• pnpm test:cityflow — 8/8
• pnpm prepare:cityflow:check — deterministic 2,601,211-byte closure
• pnpm oracle:cityflow:native — ticks 0, 73, and 251 exact within the locked float tolerance
• pnpm build:cityflow
• pnpm test:cityflow:browser — desktop and mobile, stable 200/600 DOM, zero Canvas/SVG/atlas work
• pnpm oracle:cityflow:visual — calibrated 48-frame native/browser comparison passes; native A/A and browser A/A are byte-exact
• FrameSleuth, untouched 1280×720 route: DrawFrame p50 113.421 → 19.728 ms, p95 210.782 → 34.213 ms, worst gap 232.409 → 43.047 ms, pipeline drops 864 → 175

Known limits

• The extended 252-frame diagnostic passes 250/252 frames. Frames 155–156 exceed the mean-delta threshold; worst mean delta is 1.111 against 1.0, while changed-pixel ratio remains within threshold. This draft does not claim full-cycle video parity.
• Netlify deployment wiring and a landing-page entry are intentionally not included in this draft.

[netlify]

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