fix: defer array observer reentrant notifications

[janechu]

Pull Request

Description

Fix array observer notification delivery so mutations queued while subscribers are
being notified are deferred to the update queue instead of being flushed
reentrantly when a subscriber is added.

• Prevent nested array observer notifications when a subscriber mutates the
  observed array and subscribes during notification.
• Add a Playwright regression test for mutation plus subscription during
  notification.
• Document array observation batching behavior and add a Beachball patch file.

Test Plan

• npm run build
• npm run biome:check
• npm run checkchange
• npm run build:tsc -w @microsoft/fast-element
• npm run lint -w @microsoft/fast-element
• Array observation and repeat Chromium tests: 326 passed
• npm run test currently fails in unrelated @microsoft/fast-test-harness
  Firefox fixture timeouts in this local environment.

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.