test(fast-element): verify package exports stay documented

[janechu]

Pull Request

📖 Description

Adds FAST Element package export smoke coverage for every package.json export and adds path-export docs generation/check scripts so the 3.x path exports page stays in sync with the package. The change also updates FAST Element design notes and includes a beachball change file.

📑 Test Plan

• npm run doc:path-exports:check -w @microsoft/fast-element
• npm run build:tsc -w @microsoft/fast-element
• npm run test:exports -w @microsoft/fast-element
• ./node_modules/.bin/biome check packages/fast-element/scripts/check-path-exports-docs.js packages/fast-element/scripts/smoke-package-exports.js
• npm run checkchange

✅ 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.