Include skeleton source in segmentation layer for neuroglancer links

[Copilot]

Neuroglancer links generated for materialized tables were not including the skeleton source in the segmentation layer, even when one was defined in the datastack info.

Changes

• make_seg_prop_ng_link and make_precomputed_annotation_link: Both functions now check for a skeleton_source field in the datastack info and append it to the segmentation layer's source list if present.

# Before: fixed source list
seg_layer = nglui.statebuilder.SegmentationLayerConfig(
    source=[seg_source, seg_info_source], name="seg"
)

# After: skeleton source appended when it exists
seg_sources = [seg_source, seg_info_source]
datastack_info = client.info.get_datastack_info()
skeleton_source = datastack_info.get("skeleton_source", None)
if skeleton_source:
    seg_sources.append(skeleton_source)

seg_layer = nglui.statebuilder.SegmentationLayerConfig(
    source=seg_sources, name="seg"
)

The get_datastack_info() call is zero-cost here — client.info.segmentation_source() (called just before) already populates InfoServiceClient's internal info_cache. Also cleaned up the source URL variable name from seg_layer to seg_source to remove the shadowing issue with the SegmentationLayerConfig object.