fix: update fast-element exports

[janechu]

Pull Request

📖 Description

• make the root @microsoft/fast-element export FASTElement only and publish the remaining APIs on focused subpaths
• replace import-time side effects with explicit registerFAST(), registerFASTDebugMessages(), and installPolyfills() hooks
• move hydratable ViewTemplate support into the template runtime and remove the legacy hydration installer entrypoints
• migrate internal consumers, docs, generated API pages, and size reports to the new entrypoint structure

🎫 Issues

• Fixes Update exports to be more granular #7245

👩‍💻 Reviewer Notes

• supersedes the abandoned draft in fix: keep fast-element exports explicit #7485; this version keeps the root barrel limited to FASTElement and removes the hydration installer entrypoints instead of preserving them
• @microsoft/fast-router now includes a type: none beachball change file so npm run checkchange passes alongside the existing fast-element entries
• feature work is split into three commits, followed by a merge from releases/fast-element-v3 to bring in fix: remove shared kernel #7487 and clear conflicts

📑 Test Plan

• npm run build
• npm run prebuild -w sites/website
• npm run test
• npm run biome:check
• 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.

[janechu]

Too many changes, unclear on the resolution of side effects and dead code.