fix(joint-react): useCells falls back to collection for non-graph cells

[kumilingus]

Summary

useCells(collection, …) previously resolved every member id through the GraphProvider's cell container. Cells living outside the graph (e.g. ui.Clipboard clones, freshly constructed dia.Cell instances) were silently skipped, so a selector like (cells) => cells.length === 0 always returned true regardless of how many cells the collection actually held.

Resolve cells from the collection itself when the graph container lacks them. The raw dia.Cell is converted on the fly via the existing toCellRecord mapper. A per-hook WeakMap<dia.Cell, AnyCellRecord> keeps the converted records reference-stable across renders so the outer shallow-equality short-circuit can still fire.

Per-cell change events are not subscribed for the fallback path — the only caller today (clipboard membership) needs add/remove/reset reactivity, which the collection-level subscription already covers. Adding per-cell listening is straightforward later if a use case appears.

Changes

• packages/joint-react/src/hooks/use-cells.ts — new resolveCell helper, pickCells / computeNext thread collection + WeakMap cache through; collection overload JSDoc updated.
• packages/joint-react/src/hooks/__tests__/use-cells.test.tsx — 3 new tests covering non-graph cells (records returned, length-selector reactive, ref stable across renders).

Test plan

• yarn jest --testPathPatterns=\"use-cells\" — 60 tests pass (incl. 3 new)
• Full yarn jest in @joint/react — clean
• yarn typecheck in @joint/react-plus (downstream consumer of new behavior)

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