Extract duplicated Three.js boilerplate into shared utilities

[CloudAI-X]

Summary

The skill files repeated the same setup/teardown boilerplate over and over — renderer/scene bootstrap, the requestAnimationFrame loop, resize handling, resource disposal, and the mouse→NDC conversion (the (clientX / innerWidth) * 2 - 1 line alone appeared ~8× in threejs-interaction). This PR extracts those patterns into a single shared module and points the examples at it.

New shared module lib/three-utils.js (ESM, three as a peer dep):

createRenderer(opts)                -> configured WebGLRenderer (size + clamped pixelRatio)
createScene(opts)                   -> { scene, camera, renderer }
resizeRendererToDisplay(cam, r, o)  -> resize cam/renderer/composer (+ onResize cb)
bindResize(cam, r, o)               -> window resize listener; returns unsubscribe
createRenderLoop({renderer,scene,camera,update,composer}) -> { start, stop }, Clock delta/elapsed
getPointerNDC(event, out?, element?)-> {x,y} NDC (handles touch + element rect)
disposeObject(obj, {recursive})     -> dispose geometry/material(s)/textures + detach
clampedPixelRatio(max) / getViewportSize()

Docs refactor — duplicated blocks now import the helpers instead of re-implementing them:

• threejs-interaction: all pointer→NDC conversions replaced with getPointerNDC(...).
• threejs-animation / threejs-postprocessing: Quick Start loops use createRenderLoop (the latter renders through the composer); postprocessing resize uses bindResize with an onResize callback for pass-specific resolutions.
• threejs-fundamentals: keeps its explanatory boilerplate (it's the reference) but cross-references the helpers.
• New skills/threejs-utils/SKILL.md documents the API; README.md gets a row + a Shared Utilities section.

Tooling — the repo had none. Added package.json (Prettier + node --test), test/three-utils.test.mjs (6 tests for the pure helpers getPointerNDC / disposeObject / clampedPixelRatio), a .github/workflows/ci.yml running format:check + test, and .gitignore/.prettierignore. All existing markdown was already Prettier-clean, so prettier --check . passes repo-wide.

Notes

The three version pin uses a widely-used release published well over a week ago; three is declared as a peer dependency since consumers bring their own.

Link to Devin session: https://app.devin.ai/sessions/f592e1dac0c44cc68b9bb49a2947e933
Requested by: @CloudAI-X

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring