feat(core): add board event observer

[pubuzhixing8]

Summary

• add a framework-independent createBoardEventObserver() testing helper
• record selected downstream board events while preserving the original handlers
• expose call clearing and document the required plugin ordering
• export the helper from core testing and add a patch changeset
• add a Chinese usage example covering plugin ordering, assertions, and cleanup

Why

Drawnix plugin tests need to verify that a plugin delegates an event downstream, not only that no board state changed. Keeping the observer in core makes that assertion reusable without coupling it to Jasmine, Vitest, or Jest.

Validation

• built @plait/core successfully with Angular package build
• ran the core TypeScript check successfully
• verified formatting with Prettier
• Docgeni compiled all documentation content successfully; the subsequent Angular site subprocess was terminated by SIGABRT in the local environment

No fixture-specific tests were added, per the requested scope.

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[pubuzhixing8]

Help me review @nightt5879

[nightt5879]

LGTM. The plugin ordering and downstream event delegation look correct, and the documentation is clear.