test(drawnix): add shared testing board fixture

[nightt5879]

Summary

• add an internal setupDrawnixTestingBoard() wrapper around Plait's setupTestingBoard()
• initialize a complete typed Drawnix appState, with optional pointer and creation-mode overrides
• migrate the freehand middle-button and tool-state synchronization specs to real Plait board/plugin contracts and createBoardEventObserver()
• remove broad Plait module mocks and partial board casts from the migrated tests

The helper remains under packages/drawnix/testing/ and is not exported from the published Drawnix package.

Related Issue

Closes #453

Upstream prerequisite: worktile/plait#1163 is merged, but createBoardEventObserver() has not yet been published in an npm release of @plait/core. This PR therefore remains a draft until Drawnix can update to that release.

Validation

• Applicable checks pass: lint, format check, tests, and build
• Relevant E2E and manual tests are complete or not applicable

Evidence, screenshots, or compatibility notes:

• Node.js 20.20.2 from .nvmrc
• npm run lint passes
• npm run test -- --run passes for all four workspace projects when using the local sibling Plait develop build; Drawnix: 9 files, 26 tests
• npm run build passes for all four workspace projects
• targeted oxfmt --check passes for all four changed files
• on this Windows checkout, repository-wide npm run format:check reports 127 untouched files; the pull request CI formatting step passes, and no unrelated formatting was applied
• Cloudflare Pages passes
• the current clean-install CI failure is the documented release blocker: npm @plait/core@0.93.4 does not yet provide createBoardEventObserver()
• E2E and screenshots are not applicable because this changes test infrastructure and tests only

AI Assistance

Did you use AI tools to generate or substantially modify code, tests, or documentation in this pull request?

• No
• Yes

If yes:

• Tool/model: Codex / GPT-5

• AI-assisted work: Implemented the Drawnix testing wrapper, migrated the two plugin specs, and ran the local validation commands.

• My review and validation: I reviewed and understood all four changed files before opening this pull request. The checks and results are recorded above.

• I reviewed and understand the AI-assisted changes and can explain and maintain them

Checklist

• The title and description are in English, and the change is focused
• I reviewed and understand every submitted change
• Tests, documentation, and translations were updated when needed

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View logs

[nightt5879]

@pubuzhixing8 The current CI failure is caused by the Plait dependency version: \createBoardEventObserver()\ was merged in worktile/plait#1163, but it is not available in the published @plait/core@0.93.4\ yet. This PR passes locally against the Plait \develop\ build. Could you please publish/update Plait, or let me know which version Drawnix should target? Once that version is available, I can update the Drawnix dependencies and rerun CI.

[pubuzhixing8]

@nightt5879 0.94.0 is live now!