feat: offer a three.js renderer in Code Animation with vendored addons

[atomantic]

Summary

Code Animation gains a three renderer. The host vendors the installed three.js (0.186.1) plus a fixed addon allowlist, hashes the files, and supplies the import map itself, so a film writes import * as THREE from 'three' and never reaches a CDN.

• Allowlist + hashing (server/services/codeAnimation/threeVendor.js): post-processing passes, RoundedBoxGeometry, the bloom/copy/output/bokeh shaders, RoomEnvironment, BufferGeometryUtils. The loader fails closed if any vendored module imports a relative path outside the set. Hashes land in a dependencies.json (same shape music-video documents record).
• Export: staging a three film writes the host import map (replacing any the model wrote, CDN ones included) and vendor/* next to index.html. The existing render containment already serves only files inside the staged directory with script-src 'self', so no containment change was needed.
• Preview / Download HTML: the opaque-origin sandbox cannot fetch ./vendor, so the client inlines the modules as data: URLs behind an import map (new GET /api/code-animation/vendor/three); the CSP only gains data: for scripts, and only for three films. Download HTML is a standalone file.
• Prompt: RENDERER_GUIDANCE.three teaches the deterministic one-persistent-scene renderFrame(t) pattern plus the bloom-threshold, depth-of-field and shadow-map traps from Authored Three.js music-video worlds need a cinematic post stack (depth of field, bloom, grade) #10460 / Give authored Three.js music-video worlds depth of field, bloom and a graded finish via ctx.lens #10461. Other renderers' prompts are byte-identical.

Test plan

• export.realBrowser.test.js: real Chrome runs a three film (EffectComposer + bloom + OutputPass + RoundedBoxGeometry) from the vendored files; renderFrame(0.25) called twice on the same page is pixel-identical (and differs from 0.75); the render is network-free; every vendored file's hash matches. Probed by dropping the vendor files — the test fails.
• Unit: allowlist closure and hashes, import-map replacement, export staging, prompt contract, vendor route, client import-map rewrite, preview CSP/download.
• npm run pregate green.

Closes #10464