fix(index): preserve fts prewarm position codec

[BubbleCal]

Bug Fix

What is the bug?

FTS v2 indices built with with_position=true can reconstruct cached posting lists with the wrong shared-position codec during prewarm. The prewarm path rebuilds PostingList values from projected RecordBatch objects and re-infers the positions codec from batch metadata, even though the reader has already resolved the correct partition-level positions_layout.

What issues or incorrect behavior does the bug cause?

After prewarm_index(..., with_position=True), phrase queries can fail even though the same query succeeds before prewarm. In practice this shows up as shared position stream decode failures in the cached path because PackedDelta data can be interpreted as the legacy codec.

How does this PR fix the problem?

This PR threads positions_layout from PostingListReader through the prewarm reconstruction path into CompressedPostingList::from_batch, so cached postings reuse the already-parsed shared-position codec instead of guessing from projected batch metadata. It also adds a regression test that covers the V2 + positions + tail-remainder case and verifies prewarm preserves correct phrase-query behavior.

Validation

• cargo test -p lance-index test_prewarm_with_ -- --nocapture

[codecov]

Codecov Report

❌ Patch coverage is 94.06780% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/inverted/index.rs	94.06%	6 Missing and 1 partial ⚠️

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