fix: dispose renderer-owned geometry in SparkRenderer

[ArisLiWind]

Summary

Fixes a GPU resource leak in SparkRenderer.dispose() by releasing the renderer-owned SplatGeometry created in the constructor.

Root cause

SparkRenderer constructs its own SplatGeometry:

const geometry = new SplatGeometry();
super(geometry, material);

That geometry is owned by the renderer instance, but dispose() previously released render targets, textures, workers, accumulators, and pager state without disposing the geometry itself.

Because SplatGeometry allocates both a position attribute and an index buffer, each create/dispose lifecycle could leave two WebGL buffers behind.

Fix

Store the constructor-created geometry as renderer-owned state and dispose it during SparkRenderer.dispose().

Regression coverage

Added tests to verify that:

• SparkRenderer.dispose() releases its internal geometry
• disposing a renderer does not dispose geometry assigned from outside
• dispose() is safe to call multiple times

Validation

• npm test
• npm run lint -- --reporter=summary