test: tighten multi-segment vector index coverage and cleanup

[Xuanwo]

This tightens the new multi-segment vector index path added in #6220.

It enforces disjoint fragment coverage when committing a segment set, adds regression coverage that grouped segment coverage matches the union of its source shard coverage, and verifies that remap only touches segments covering affected fragments.

It also adds cleanup coverage for both replaced committed segments and stale uncommitted _indices/<uuid> artifacts, and documents these contracts in the distributed indexing guide.

[github-actions]

Review

Clean PR — tests are well-structured and the validation logic is correct.

One minor suggestion

Fragment overlap check could use is_disjoint instead of intersection_len (rust/lance/src/index.rs):

// current
if covered_fragments.intersection_len(segment.fragment_bitmap()) > 0 {

// slightly more readable
if !covered_fragments.is_disjoint(segment.fragment_bitmap()) {

is_disjoint can short-circuit on the first shared element, whereas intersection_len must count all overlapping elements. In practice the difference is negligible for small bitmaps, but is_disjoint communicates intent more clearly and is never slower.

Everything else looks good

• Disjoint coverage validation at commit time is the right place for it.
• Cleanup tests correctly verify both replaced-segment removal and uncommitted-artifact removal.
• Remapper test properly asserts segment-granular behavior (only affected segments get remapped).
• Fragment coverage assertion in the grouped-segment build test is a good regression guard.
• Doc updates are accurate and concise.

LGTM with the optional nit above.