P4: evaluate bounded Canvas render density without changing 2D defaults

[nekomario28]

Follow-up to #276 P4.

Evidence / problem

The Canvas2D viewers currently size backing stores directly from window.devicePixelRatio || 1, while the Three.js renderer already bounds render density (mobile auto = 1x, desktop auto <= 1.45x, high <= 1.8x, low = 0.85x). On DPR3 hardware, native Canvas backing-store area is roughly 9x CSS pixel area.

Current Canvas source census with direct DPR handling:

• public-viewer.js
• public-radial-viewer.js
• public-tree-viewer.js
• public-treemap-viewer.js
• public-timeline-viewer.js
• public-cluster-viewer.js
• public-sunburst-viewer.js
• public-tree-router.js

Safe first increment

Do not change the 2D default yet.

1. Add one browser-side render-density policy helper.
2. With no render query, return native DPR exactly (current behavior).
3. Experimental URL-only modes:
   • render=auto: mobile <= 1x, desktop <= 1.45x
   • render=high: mobile <= 1.25x, desktop <= 1.8x
   • render=low: <= 0.85x
4. Apply it to all Canvas runtimes through one build postprocessor rather than duplicating policy code.
5. Keep render renderer-local and outside transferable 2D/3D state.
6. Add DPR3 Chromium evidence proving CSS geometry is unchanged while backing-store area is bounded for opt-in auto.
7. Keep a native-mode regression proving absent render preserves current DPR3 backing-store behavior.
8. Only after visual/readability/performance evidence decide whether Auto should become a user-facing/default mode.

Non-goals

• no P3 Activity/Quality/semantic-edge work
• no immediate 2D default-quality reduction
• no layout/hit-test changes
• no addition to Spatial Core

Refs #276.

[nekomario28]

P4 implementation evidence from PR #286 final head cbafb0f2f5dcf74fdf4fe57bdb50e290f764c144:

• no render query preserves native DPR exactly;
• DPR3 shared desktop native: CSS 1280×553, backing 3840×1659;
• explicit render=auto: CSS 1280×553 unchanged, backing 1856×802 at 1.45×;
• measured backing-store area ratio = 0.2336548121, a 76.6345% reduction;
• dedicated Radial also resolves Auto to ~1.45×;
• DPR3 mobile Auto resolves to 1× with CSS 390×421.609, backing 390×422, and no horizontal overflow;
• full Verify / Chromium / iPhone WebKit are green.

Keep this issue open after #286 merges: changing the 2D default or exposing a UI control still needs visual/readability/performance evidence, especially Canvas text at reduced backing density.