fix: split index_statistics to reduce rustc query depth

[Xuanwo]

This refactors DatasetIndexExt::index_statistics into smaller helper functions to reduce the async state-machine complexity.

This avoids rustc query depth overflow when compiling with nightly + coverage instrumentation and many enabled features.

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Parts of this PR were drafted with assistance from Codex (with gpt-5.2) and fully reviewed and edited by me. I take full responsibility for all changes.

[github-actions]

Code Review

This is a well-structured refactoring that addresses the rustc query depth overflow issue when compiling with nightly + coverage instrumentation.

Summary

The PR successfully breaks down the large index_statistics async function into smaller helper functions:

• sum_indexed_rows_per_delta - synchronous row counting
• unique_indexed_fragment_count - synchronous fragment deduplication
• serialize_index_statistics - JSON serialization helper
• migrate_and_recompute_index_statistics - migration handling
• index_statistics_frag_reuse / index_statistics_mem_wal - special index handlers
• index_statistics_scalar - main scalar index statistics
• collect_scalar_indices_statistics - per-index statistics collection
• fragment_statistics_or_migrate - fragment statistics with migration fallback

Verification

The behavior is preserved correctly. I noted that the original code tracked index_typename but never used it in the output JSON (only index_type was used), so its removal is appropriate cleanup.

LGTM ✓

[westonpace]

Thanks for investigating into this. I saw the CI error and had no idea what to do 😆. I have a few naming nits but they aren't absolutely required.

[codecov]

Codecov Report

❌ Patch coverage is 73.24841% with 42 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/index.rs	73.24%	29 Missing and 13 partials ⚠️

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