[Remote rendering 3.5] Server-authoritative color variable lookup tables

Start with RuntimeAppState and the current client-owned LUT path on main, then compare it with the round-trip spike branch and the mapper/lookup-table responsibilities. Move component selection with SelectColorArray(name), VectorMode, and VectorComponent while checking the known framework cache behavior. Done means the server-applied LUT survives refresh or reconnect and produces no visual differences from main.

📝 Description of the feature

Intent: The lookup table backing each color variable, the color mapping itself rather than a part's reference to it, becomes server-owned. This is the last piece of the state to invert.

Context: Story 3.1 makes each part's reference server-authoritative: which color variable it uses, which component, over what range. the lookup table that reference resolves against still lives client-side on the wasm path, so the server cannot reconstruct what a scene actually looks like, and a user-modiifed color mapping is lost on reload. RuntimeAppState reserves a slot for this; this story fills it.

Also move component selection from the mapper to the lookup table (SelectColorArray(name) plus VectorMode / VectorComponent), which is required for magnitude coloring of vector variables. The serialized reference can stay as it is, since component: -1 already means magnitude, but decide whether to keep that sentinel or mirror VTK's two-field shape.

Known risk: the lookup table sync issue: On the round-trip spike branch, the LUT state failed to reach the renderer correctly: the framework's client-side cache applied a stale snapshot instead of the state the server delivered. It was diagnosed as a framework-level issue, with no viable local workaround, and has not been retested since.

What kept this from being an issue on main is that the client rebuilds its own LUT on every color change, which creates a fresh object (and object ID) every time, and the framework's stale cache is keyed on object ID. This story removes that, so this is the story that creates that condition; it is therefore expected here. See the first comment below for more details on diagnosing the issue.

If we do encounter a similar issue in this user story, it can be deferred until 6.2 [to add link to user story once it's created] - where we retest after bumping the vtk-wasm and trame-vtklocal versions to be current (we are currently a major version behind on both dependencies). Defer by leaving the client owning its own LUT on the wasm path, which is what main does today. The spike branch tried making the server own the LUT state and adding a client-side reapply to compensate; this did not reliably work, so is not a recommended stop-gap.

The remote rendering work that follows in Phase 4 and Phase 5 is not dependent on the present user story.

Acceptance Criteria

• Color LUT is applied on the server side, so on a refresh or reconnect, the LUT is preserved.
• No in-visualizer differences from main.

💵 Business Value

No response

🔗 Useful links and references

No response