[Remote rendering 4.1] Add headless RemoteRenderer and RCA streaming backend

### 📝 Description of the feature

Stand up the server-side rendering path on the backend:
* a `RemoteRenderer` that implements `IRenderer` against an off-screen `vtkRenderWindow` (`SetOffScreenRendering(1)`), writing per-part mutations through the shared `VtkNodePipeline` and owning camera state directly on the server-side camera (unlike the local path, where the camera lives in the browser).
* `RemoteApp` wires the render window into trame-rca so frames can stream to the client, and complete `VisorRemoteScene` against the runtime-state hooks from user story 1.2. This includes render coalescing (back to back mutations produce one frame).
* This is the largest story in the phase and is expected to break down into several sub-PRs, e.g.
* The off-screen renderer
* the streaming wiring
* coalescing + camera push

Local mode is untouched in this user story; all of this new code is behind the mode flag.

### Acceptance Criteria

* A connecting client receives an initial frame.
* Tests pass

### 💵 Business Value

_No response_

### 🔗 Useful links and references

_No response_

Trace the RemoteRenderer, RemoteApp, VtkNodePipeline, and VisorRemoteScene entry points described in the issue, starting with the existing local renderer and runtime-state hooks. Break the work into the off-screen renderer, streaming wiring, and coalescing/camera behavior; done means a connecting client receives an initial frame and the tests pass.

Vom Indexierungsmodell aus dem Issue-Text verfasst.