Support Shared Tileset Reference for Combined Vector & Geometry Tilesets

Currently, only tilesets containing exclusively vector data are supported. In the future, tilesets may contain both vector and non-vector geometry. To support use cases where a vector tiles overlay is draped over the same tileset as regular geometry, ensure the overlay logic can reference and share the underlying tileset instead of always creating a separate instance.

Start by tracing the overlay logic and where tileset instances are created for vector and regular geometry data. Confirm how a combined tileset can expose one shared underlying instance, then verify that vector overlays reuse it rather than creating a separate tileset.

Written by the indexing model from the issue text.