fix(joint-react): StrictMode duplicate feature registration and resetCells ghost cells under React 18

[samuelgja]

The library supports React 18+, but development and CI ran only on React 19, which hid two React 18 bugs. This PR fixes both and adds a React 18 test run so they stay fixed.

Fixes

Duplicate paper feature under React 18 StrictMode (selection "2x drag" bug).
useCreateFeature registers paper-target features synchronously during render. React 18's StrictMode invokes the render body twice, and ref writes from the discarded first pass are not carried into the second — so featureRef.current alone could not dedupe, and a second feature instance was constructed. The duplicate's constructor bound paper listeners that were never cleaned up (only the committed instance's clean() runs), so every interaction fired twice — e.g. a selection drag translated each cell by 2x. The hook now adopts an instance already registered in the store (external state, which StrictMode does preserve) instead of constructing a duplicate.

Ghost cells after resetCells().
graph.resetCells() emits a bulk reset containing only adds for the surviving cells — no per-cell remove events. The reactive container therefore kept cells the reset had dropped, and useCells() reported ghost cells the canvas no longer showed. graph-changes now flags reset batches (isReset), and graph-projection prunes any container cell not present in the reset's surviving set, mirroring the reconciliation the React-driven updateGraph() path already does.

Testing

• New jest.react18.config.mjs runs the full suite with React resolved to an aliased React 18 install (react18 / react18-dom dev deps); the React-19-specific react-compiler project is skipped.
• yarn test now runs both test:react19 (default) and test:react18.
• Regression tests: a StrictMode listener-leak test for the feature duplication (passes trivially on React 19, bites on 18) and use-reset-cells-ghost.test.tsx covering both the useCells count and the projection container after resetCells.