feat(fast-element): warn on duplicate render registrations

[janechu]

Pull Request

📖 Description

Adds a debug warning when RenderInstruction.register() replaces an existing instruction for the same model type and instruction name. The PR adds focused Playwright coverage for duplicate and distinct-name render registrations, documents replacement behavior in packages/fast-element/DESIGN.md, and includes a beachball change file.

📑 Test Plan

• npm run build:tsc -w @microsoft/fast-element
• FAST_TEST_SERVER_URL=http://localhost:5273/ npx playwright test src/templating/render-registration.pw.spec.ts --project=chromium
• npx biome check packages/fast-element/src/debug.ts packages/fast-element/src/interfaces.ts packages/fast-element/src/templating/render.ts packages/fast-element/src/templating/render-registration.pw.spec.ts packages/fast-element/DESIGN.md change/@microsoft-fast-element-4dbc9ebd-2055-4ad1-af2a-5f3dacb6decc.json
• 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.