feat(freehand): extract standalone plugin package

[nightt5879]

Summary

• extract the reusable freehand engine into the standalone @plait-board/freehand package
• keep Drawnix-specific presets, gesture blocking, and laser eraser trails in a small adapter injected through plugin options
• add Nx build/publish configuration, focused behavior tests, and package documentation

Testing

• npx nx run-many -t=test --all -- --run
• npm run build
• npm run lint
• npm run format:check
• npx nx build web
• npx tsc -p packages/freehand/tsconfig.lib.json --noEmit
• npx tsc -p packages/drawnix/tsconfig.lib.json --noEmit
• clean npm pack / install / require('@plait-board/freehand') consumer smoke test

A full all-project TypeScript check still reports the existing CSS custom-property typing errors in the unchanged island.tsx and stack.tsx files; the new package and Drawnix library type checks pass.

AI-assisted contribution

• Tool: OpenAI Codex
• Used for package-boundary design, refactoring, tests, and documentation
• I reviewed the resulting diff and verified it with the commands above; I understand and can maintain the submitted changes

Closes #464

[cloudflare-workers-and-pages]

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View logs

[nightt5879]

Closing because this implementation was started prematurely during a planning discussion. No implementation decision was intended.