feat: Remote rendering 3.1a - add per-part write path to the dataset registry

[LKasianAnsys]

Issue

Addresses #18

Context

This is the first 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 first step: adding registry-side storage and a write path, with no callers yet. The VTK operations that apply the state land in PR 2, the trigger handlers in PR 3, the client-initiated triggers in PR 4, and save/load state in PR 5.

Notes:

• The per-part visual state lives on the dataset registry, alongside the rest of the per-part metadata get_scene_details builds its payload from.
• When calling the setters (set_part_visibility, set_part_opacity, etc), if a part record is not found, one is created.

Summary

This pull request adds a new per-part write path to the VisorDatasetRegistry, enabling fine-grained control and mutation of individual part states within datasets at runtime. It introduces a set of methods for getting and setting properties (such as visibility, opacity, color, and selection) on a per-part basis, along with unit tests to ensure correct behavior. The changes also clarify the distinction between name-keyed and id-keyed state management paths.

Per-part state management enhancements:

• Added multiple methods to VisorDatasetRegistry for per-part state mutation and retrieval, including set_part_visibility, set_part_opacity, set_part_diffuse_color, set_part_selected, set_part_color_variable, clear_part_color_variable, and replace_part_states. These methods allow direct manipulation of runtime state for individual parts, with robust handling for unknown part IDs and upsert logic for missing state records.
• Introduced find_dataset_id_for_part and get_part_state methods to locate the owning dataset for a part and retrieve its current state, supporting the new per-part write path.

Tests:

• Added a suite of unit tests to cover all new per-part write path methods, including edge cases for unknown parts, upsert behavior, and atomic state updates. Also introduced a make_part_dataset helper for building test datasets with real runtime state. [1] [2]