feat: support zonemap index segments

[beinan]

Summary

• allow zonemap to build uncommitted segments over explicit fragment subsets
• add a logical multi-segment zonemap index that unions segment search and fragment coverage
• route name-based scalar zonemap loading through the logical multi-segment path in query execution

Context

This keeps the public index type as zonemap and follows the index-segment direction discussed in the zonemap vote, instead of introducing a separate public partitioned zonemap type.

Testing

• cargo test -p lance-index test_null_zonemap_index --lib
• cargo test -p lance test_open_named_scalar_index_uses_all_zonemap_segments --lib
• cargo test -p lance test_like_prefix_with_zone_map --lib

[claude]

Claude Code Review

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[codecov]

Codecov Report

❌ Patch coverage is 76.45503% with 89 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/index/scalar_logical.rs	69.62%	74 Missing and 8 partials ⚠️
rust/lance/src/dataset/scanner.rs	96.15%	2 Missing and 2 partials ⚠️
rust/lance/src/io/exec/scalar_index.rs	25.00%	2 Missing and 1 partial ⚠️

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