Add runtime scene attribute updates

[ruziniuuuuu]

Description

Adds a renderer-neutral RenderContext.update_scene_attribute(...) API for targeted non-physics scene state such as lights and material asset inputs. Renderers that consume live USD keep the base no-op behavior; backends with a private scene representation can synchronize the update explicitly.

The OVRTX implementation supports immediate updates in both legacy and ovstage modes. Updates submitted before stage initialization are copied, queued, and flushed after stage load and environment cloning. The ovstage path also preserves USD asset-path semantics for HDR and material inputs.

Fixes #6943

Type of change

• New feature (non-breaking change which adds functionality)

Screenshots

Not applicable; this is a renderer API and synchronization contract change.

Validation

• uv run --frozen isaaclab -f
• .venv/bin/pytest source/isaaclab/test/renderers/test_simulation_render_context.py -q — 13 passed
• PYTHONPATH=source/isaaclab:source/isaaclab_ov <ovrtx-env>/bin/pytest source/isaaclab_ov/test/test_ovrtx_renderer_contract.py -q — 26 passed with OVRTX installed
• .venv/bin/python tools/changelog/cli.py check develop
• Real ovstage probe wrote numeric and ASSET_STRING attributes and advanced the write floor successfully

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with uv run --frozen isaaclab -f
• I have documented the public API with docstrings
• My changes generate no new warnings
• I have added tests that prove the feature works
• I have added a changelog fragment for every touched package
• I have added my name to CONTRIBUTORS.md