fix(fast-element): add update scheduler fallback

[janechu]

Pull Request

📖 Description

Adds a setTimeout fallback when requestAnimationFrame is unavailable for async Updates scheduling while preserving browser batching through requestAnimationFrame. The PR documents scheduler behavior and updates FAST Element change and size artifacts.

📑 Test Plan

• npm ci --no-audit --no-fund
• npx playwright test "src\\observation\\update-queue.pw.spec.ts" --project=chromium --reporter=line
• npm run build -w @microsoft/fast-build
• npm run build -w @microsoft/fast-element
• node_modules\\.bin\\biome.cmd check packages\\fast-element\\src\\observation\\update-queue.ts packages\\fast-element\\src\\observation\\update-queue.pw.spec.ts packages\\fast-element\\DESIGN.md packages\\fast-element\\MIGRATION.md packages\\fast-element\\docs\\architecture\\updates.md sites\\website\\src\\docs\\3.x\\resources\\browser-support.md change\\@microsoft-fast-element-a2febdf1-f514-4603-8aaa-8ac580c5e3ad.json
• npm run checkchange

Note: npm run biome:check exits 1 on Windows because the repo script invokes the POSIX shim at node_modules/.bin/biome; direct biome.cmd check on changed files passed.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.