feat: Remote rendering 3.1b - implement per-part apply logic on the renderer and node pipeline

[LKasianAnsys]

Issue

Addresses #18

Context

This is the second of 5 PRs for user story 3.1 (#18) of the phased implementation plan (here) for adding remote rendering in VISOR.

Per-part visual state (visibility, opacity, diffuse color, color variable), is currently held client-side on VisorPartState and reaches the server only via getAppStateAsync when the save_state API is called. This story inverts that, making VisorDatasetRegistry the source of truth for per-part state by the end of 3.1.

This PR is the second step: filling in the no-op apply_* logic on the renderer so the server can drive VTK for a single part. There are still no callers in this PR. The trigger handlers are added on the server in PR 3, the client-initiated triggers are added in PR 4, and this is extended to save/load state in PR 5.

Notes:

• Per-node mutation lives on VtkNodePipeline. The renderer resolves the node and delegates, and does no VTK work of its own. As much as possible, the VTK logic common between the LOCAL (vtk-wasm) and REMOTE modes will be shared.
• set_color_variable does array validation and mapper calls only. Moving color variable ownership color LUT authority and application on the server is scheduled for user story 3.5, so this is deferred here.
• An unrecognized point/cell association logs and returns rather than falling through to a default.

Copilot summary

This pull request implements per-part apply logic for the renderer and node pipeline, enabling fine-grained visual control over individual parts in the VTK pipeline. The main changes introduce concrete implementations for visibility, opacity, diffuse color, selection, and color variable application at the part level, moving the logic from no-ops to actual delegation to the VtkNodePipeline.

Renderer and Node Pipeline Enhancements:

• Implemented per-part apply logic for apply_visibility, apply_opacity, apply_diffuse_color, apply_selected, apply_color_variable, and clear_color_variable in VisorLocalRenderer, delegating to corresponding methods on VtkNodePipeline. These methods now resolve the pipeline for a given node ID, log a debug message if the node is unknown, and never raise exceptions. (src/ansys/visor/viewer/renderer/local_renderer.py) [1] [2]
• Added new methods to VtkNodePipeline for set_selected, set_visibility, set_opacity, set_diffuse_color, set_color_variable, and clear_color_variable, with detailed docstrings and robust handling of invalid associations or missing arrays. (src/ansys/visor/viewer/vtk/node_pipeline.py)

Testing Improvements:

• Refactored and expanded unit tests in test_local_renderer.py to verify that each per-part apply method delegates correctly to the pipeline, including handling of unknown node IDs as logged no-ops. Added a new test class TestDelegatedApplyBodies for this purpose. (tests/unit/renderer/test_local_renderer.py)
• Added a new fixture and supporting code in test_node_pipeline.py to enable testing of color variable application with realistic VTK datasets containing named point and cell arrays. (tests/unit/vtk/test_node_pipeline.py)

These changes collectively enable fine-grained, per-part visual control in the renderer, laying groundwork for further features.

The base branch was changed.

[LKasianAnsys]

@margalva Thank you for approving; after the 3.1a merge this branch was updated to the new base (main), and needs a re-approval.