feat(index): support distributed LabelList scalar index builds

[jackye1995]

Summary

LabelListIndexPlugin::train_index unconditionally rejected fragment-scoped
training (fragment_ids.is_some()), so a LabelList index could not be built
through the distributed / segmented path (per-fragment execute_uncommitted +
merge_existing_index_segments) the way BTree, Inverted, Bitmap, and FM already
are. This makes LabelList a first-class distributed scalar index.

Changes

• lance-index/src/scalar/label_list.rs: train_index ignores fragment_ids and
  builds over the already fragment-scoped stream (mirrors FMIndexPlugin); a partial
  index over a fragment subset is correct since it covers exactly those rows. Add
  merge_label_list_indices, which unions the per-segment bitmap states and the
  list_nulls row sets. LabelList wraps a BitmapIndex plus a null-row set and
  distributed segments cover disjoint rows, so this is a cheap union (the same
  operation LabelListIndex::update performs) — no source re-scan. Mirrors
  merge_bitmap_indices but also carries list_nulls.
• lance/src/index/scalar/label_list.rs (new): merge_segments opens each source
  segment as a LabelListIndex and calls merge_label_list_indices (mirrors
  scalar/bitmap.rs).
• lance/src/index.rs: merge_existing_index_segments routes all_label_list
  segments (details LabelListIndexDetails) to the new merge; add
  segment_has_label_list_details.
• lance/src/index/scalar.rs: declare the label_list module.

Covered by test_label_list_merge_existing_index_segments: one LabelList segment per
fragment, merged via merge_existing_index_segments, answers array_has_any across
both fragments with the same row count as a pre-index full scan.

[codecov]

Codecov Report

❌ Patch coverage is 82.25806% with 33 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/label_list.rs	69.81%	8 Missing and 8 partials ⚠️
rust/lance/src/index/scalar/label_list.rs	67.34%	9 Missing and 7 partials ⚠️
rust/lance/src/index/create.rs	98.71%	1 Missing ⚠️

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