BPMN W9b: the React Flow adapter (gated on React Flow stabilising)

Part of elsa-workflows/elsa-core#7909 (W9b). **Gated on React Flow stabilising, not on any BPMN milestone.** Filed so the gap is visible, not so it gets scheduled.

The React Flow adapter for the canvas-neutral BPMN view model (W9a). Ports closely from Elsa 4 Studio's `buildBpmnCanvas` in `bpmnAdapter.ts`, which is the canvas-specific half of the same split.

Where: beside `src/modules/Elsa.Studio.Workflows.Designer/ClientLib/src/react-designer`. Same read-only scope as W9c, same rendering fixtures, same events. `BpmnDesignerWrapper` (W10) already switches on `DesignerOptions.UseReactFlow`; this item fills the branch that currently shows a notice.

Do not start this while React Flow is "too buggy to be the default" (maintainer statement recorded on #7909). The W9a boundary exists precisely so this can be written later without redoing any BPMN logic.

Wait for React Flow to stabilise, then read Elsa 4 Studio's bpmnAdapter.ts and the canvas-neutral BPMN view model boundary from W9a. Start beside src/modules/Elsa.Studio.Workflows.Designer/ClientLib/src/react-designer and inspect BpmnDesignerWrapper's React Flow branch, along with the shared rendering fixtures and events. Done means the read-only adapter fills the branch that currently shows a notice.

Written by the indexing model from the issue text.