fix: replace arrays during observerMap deepMerge

[janechu]

Pull Request

Description

Fix observer-map-managed array updates so deepMerge replaces arrays instead of
mutating existing observed arrays in place.

• Replace arrays during observerMap deep merges to avoid synchronous reentrant
  array notification work.
• Add a Playwright regression test for the observerMap + deepMerge reentrant
  notification path.
• Document the array replacement behavior and add a Beachball change file.

Test Plan

• npm run build -w @microsoft/fast-html
• npm run build:tsc -w @microsoft/fast-html
• npm run lint -w @microsoft/fast-html
• npm run test:chromium -w @microsoft/fast-html -- test/fixtures/deep-merge/deep-merge.spec.ts
• 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.