fix(fast-element): improve hydration mismatch diagnostics

[janechu]

Pull Request

📖 Description

Improves hydration mismatch behavior by falling back to client rendering when render() hydration has no SSR view boundaries and reconciling repeat() hydration item-count mismatches by creating missing client views or removing extra SSR ranges. The PR adds targeted hydration coverage and documents the mismatch behavior.

📑 Test Plan

• npm run build:tsc -w @microsoft/fast-element
• npm run build:rollup -w @microsoft/fast-element
• npx biome check packages\fast-element\src\templating\render.ts packages\fast-element\src\templating\repeat.ts packages\fast-element\src\components\hydration.pw.spec.ts packages\fast-element\DESIGN.md change\@microsoft-fast-element-15f98c55-f4bb-4b85-9c55-564b10cdd8c5.json
• npx playwright test hydration.pw.spec.ts --project=chromium --config=playwright.hydration-diagnostics.config.ts (isolated local server on port 5178)
• 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.