Add opt-in bounded render density to Canvas viewers

[nekomario28]

Summary

Starts #276 P4 / #285 by donating the Three.js render-density policy to Canvas2D without changing current 2D defaults.

Compatibility boundary

• no render query (or an unknown value) keeps the existing native devicePixelRatio exactly;
• the new modes are URL-only experimental controls, not a new visible UI or a new transferable semantic state;
• layout, hit testing, pan/zoom, labels and renderer semantics remain in CSS-pixel coordinates; only the Canvas backing-store ratio changes when explicitly opted in.

Experimental modes

• render=auto: mobile <= 1x, desktop <= 1.45x
• render=high: mobile <= 1.25x, desktop <= 1.8x
• render=low: <= 0.85x

These mirror the established Three.js donor policy.

Shared implementation

• one render-density.js browser helper owns the policy;
• one build postprocessor patches the eight Canvas runtimes that currently use direct DPR:
  • shared viewer.js
  • Radial / Tree / Treemap / Timeline / Cluster / Sunburst
  • shared tree-router.js resize synchronization
• the policy bootstrap is attached only to the seven pages that host those Canvas runtimes;
• the postprocessor fails closed unless each target still has exactly one native DPR assignment.

Evidence / gates

• unit tests prove native compatibility, Auto/High/Low caps, invalid-value fallback, all-eight-runtime coverage and idempotence;
• DPR3 Chromium evidence compares native vs explicit Auto on the shared 2D viewer, checks dedicated Radial, and verifies mobile Auto at 1x;
• Auto must preserve CSS canvas geometry while reducing DPR3 desktop backing-store area to less than one quarter of native;
• full existing 2D/3D browser regression remains enabled.

This PR intentionally does not make Auto the default. A default/UI decision stays blocked on visual/readability/performance evidence.

Refs #285 #276.

[nekomario28]

Final exact-head evidence for cbafb0f2f5dcf74fdf4fe57bdb50e290f764c144:

• Verify: success
• Chromium browser E2E: success
• iPhone WebKit smoke: success
• artifact: browser-e2e, id 9610330557, sha256 57269172db709e3d867274f0f55ff5a68de4deb6c4ecf74aed7c2ef6efc4ea26

Persisted DPR3 Canvas evidence:

• shared desktop native: CSS 1280×553, backing 3840×1659, ratio 3×
• shared desktop render=auto: CSS 1280×553 unchanged, backing 1856×802, policy 1.45×
• measured backing-store area ratio: 0.2336548121 (76.6345% reduction)
• Radial desktop Auto: CSS 1280×571, backing 1856×828, ~1.45×
• shared mobile DPR3 Auto: CSS 390×421.609, backing 390×422, ~1×; document scroll width = viewport width = 390

This remains opt-in only; native DPR stays the default. Default/user-facing Auto remains blocked on visual/readability/performance evidence.