Centralize Canvas and SVG plot geometry

[happykhan]

Summary

• introduce one renderer-independent plot scene for track visibility, radial layout, arc geometry, colours, tooltips, scale markers, and legends
• reduce Canvas and SVG implementations to backend-specific painters consuming that scene
• use the same presentation selector for the preview and exports
• enforce the shared-scene boundary in the architecture check
• add contract and SVG structure coverage for the shared scene

Why

Canvas preview and SVG export previously recalculated the plot independently. That allowed behaviour to drift whenever a control or renderer feature changed. This is stacked on #42 so the immediate GC export correction remains small and independently reviewable.

Verification

• npm run lint
• npm run quality:architecture
• npm run quality:licenses
• npm test (181 passed)
• npm run build
• npm run test:e2e (25 passed, with the session-preservation assertions from the next stacked commit included locally)
• npm run test:e2e:dev (1 passed)

A direct npm audit currently reports pre-existing transitive nanoid advisory 1139427. This PR does not change dependencies. GitHub CI is configured only for PRs whose base is master, so this stacked PR receives the Cloudflare build check now and will receive the full workflow when retargeted after its prerequisite is merged.

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