feat: add configurable FTS index prewarm options

[BubbleCal]

Summary

• add PrewarmOptions and FtsPrewarmOptions on the Rust side, with dataset plumbing for prewarm_index_with_options
• add Python prewarm_index(..., *, with_position=False) support for FTS indices while keeping the default prewarm path unchanged

[github-actions]

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[github-actions]

Code Review

Overall this is a well-structured PR. The approach of separating position data into a dedicated cache during prewarm — so phrase queries avoid a lazy re-read — is sound. The cache flow is correct: take_positions() strips positions from the posting list before caching it, stores them under PositionKey, and read_positions() uses get_or_insert_with_key which naturally hits that pre-populated entry.

Test coverage is solid: both Rust and Python tests verify the cache-entry-count invariant (no growth after prewarmed phrase query), the validation error for no-position indices, and the Python keyword-only argument enforcement.

Minor observations (non-blocking)

1. Stale comment — index.rs:1674 still says "when the cache was populated, the positions column was not loaded". After this PR, the positions may also be absent because they were intentionally extracted during prewarm. Consider updating the comment to reflect both paths.

2. _ => arm in prewarm_index_with_options (rust/lance/src/index.rs) — Since PrewarmOptions is #[non_exhaustive] with a single variant, the wildcard is required for compilation but currently unreachable. Fine for forward-compat, just noting it's dead code today.

No P0/P1 issues found. LGTM.

[codecov]

Codecov Report

❌ Patch coverage is 91.80328% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/inverted/index.rs	92.45%	6 Missing and 2 partials ⚠️
rust/lance/src/index/api.rs	0.00%	2 Missing ⚠️

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