Add injectable LOD runtime interfaces

[promontis]

Add injectable runtime seams for Spark's LOD and sorting pipeline via SparkRendererOptions, while preserving current behavior by default.

This PR exposes three extension points:

• ILodTraverser for LOD tree traversal
• ISortProvider for depth sorting
• ILodScheduler for deciding how overlapping LOD update requests are handled

Default behavior remains unchanged:

• WorkerLodTraverser remains the default traversal implementation
• WorkerSortProvider remains the default sort implementation
• DropIfBusyLodScheduler preserves Spark's current "drop overlapping LOD updates while busy" behavior

Why ILodScheduler is separate:

• traversal and sorting describe how work is performed
• scheduling describes when overlapping LOD work should run, be skipped, or be coalesced
• keeping scheduling as a separate seam avoids baking policy into SparkRenderer while keeping the default path intact

This keeps the public change narrowly scoped and non-breaking while enabling external integrations to inject custom policies, such as:

• latest-camera-wins LOD scheduling
• custom GPU/WebGPU traversal or sorting paths
• domain-specific residency or prioritization logic outside Spark core

The interfaces and default implementations are exported from the package root so consumers do not need internal module-path imports.

Validation:

• existing lint/test hooks pass
• added a focused test covering DropIfBusyLodScheduler default behavior