fix(index): use global BM25 scorer for WAND pruning

[BubbleCal]

Bug Fix

What is the bug?

Partition WAND candidate generation used partition-local BM25 statistics while final indexed FTS scoring used the global BM25 scorer. Because WAND also shares a top-k floor across partitions, one partition could publish a threshold in a different score space and cause another partition to prune a candidate that should win under the global scorer.

What incorrect behavior does this cause?

Multi-partition FTS queries could drop the correct global top-k result when local term distributions differ from global term distributions.

How does this PR fix the problem?

• Materializes one active MemBM25Scorer for each indexed BM25 search and threads it through posting loading, WAND scoring, and final aggregation.
• Computes posting query weights from the active scorer instead of partition-local IDF.
• Recomputes compressed posting block upper bounds in the active scorer's score space before WAND pruning.
• Uses a conservative BM25 ceiling for plain postings without usable scorer-space impacts.
• Removes the obsolete partition-local IndexBM25Scorer path.
• Adds a regression test where a shared threshold previously pruned the row that wins under global BM25.

Linear: https://linear.app/lancedb/issue/OSS-1345/use-global-bm25-scorer-for-partition-wand-pruning

Validation

• cargo fmt --all --check
• CARGO_TARGET_DIR=/tmp/lance-target-oss-1345 cargo test -p lance-index test_bm25_search_shared_threshold_keeps_global_bm25_winner -- --nocapture
• CARGO_TARGET_DIR=/tmp/lance-target-oss-1345 cargo test -p lance-index scalar::inverted::wand::tests -- --nocapture
• CARGO_TARGET_DIR=/tmp/lance-target-oss-1345 cargo test -p lance io::exec::fts::tests::test_match_query_exec_with_base_scorer_matches_baseline -- --nocapture
• CARGO_TARGET_DIR=/tmp/lance-target-oss-1345 cargo test -p lance-index fuzzy_and -- --nocapture
• CARGO_TARGET_DIR=/tmp/lance-target-oss-1345 cargo clippy -p lance-index -p lance --tests -- -D warnings

[codecov]

Codecov Report

❌ Patch coverage is 94.79167% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/inverted/wand.rs	86.44%	7 Missing and 1 partial ⚠️
rust/lance-index/src/scalar/inverted/index.rs	98.42%	2 Missing ⚠️

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