perf(fast-element): filter late attribute observation

[janechu]

Pull Request

📖 Description

Filters late-attribute MutationObserver callbacks to only attributes in the late attribute lookup while preserving late-defined attribute sync/update behavior. The PR adds focused Playwright coverage for late-defined attribute syncing and unrelated attribute changes, updates design notes, and includes a change file.

📑 Test Plan

• playwright test element-controller.pw.spec.ts --project=chromium --grep "late-defined attributes"
• npm run build:tsc -w @microsoft/fast-element
• beachball check --branch origin/releases/fast-element-v3

✅ 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.