fix(index): use exact scalar indices for filtered count_rows

[myandpr]

Fixes #2682.

Summary

• add an exact scalar-index fast path for Dataset::count_rows(Some(filter))
• keep the existing scanner path as the fallback when the filter is not an exact index search
• load and query all physical segments for a logical scalar index name
• share scalar-index fragment coverage logic between count / scanner / exec paths

Problem

Dataset::count_rows(Some(filter)) always went through the scan-based count path, even when the filter could be satisfied exactly by a scalar index. That meant we still materialized row ids and counted them through the scanner.

There was also a correctness gap once a logical scalar index had multiple physical segments. Coverage checks could treat the logical index as fully covering the dataset, while query execution still loaded only one segment. In that state, an exact result could be treated as complete even though it only came from part of the logical index.

Approach

This change adds a dedicated exact-count path for filtered count_rows and only uses it when the filter planner produces an exact scalar-index search with full fragment coverage and no refine step. Exact allow-list and block-list results are counted directly, with deletion masks handled explicitly.

To keep coverage checks and execution aligned, scalar-index loading now aggregates all segments for the same logical index name into a LogicalScalarIndex and unions their search results.

Tests

• cargo test -p lance test_fast_count_rows -- --nocapture
• cargo test -p lance test_logical_scalar_index_search_unions_all_segments -- --nocapture
• cargo test -p lance test_plans -- --nocapture

[codecov]

Codecov Report

❌ Patch coverage is 48.81657% with 173 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/io/exec/scalar_index.rs	43.33%	82 Missing and 3 partials ⚠️
rust/lance/src/dataset/scalar_count.rs	50.31%	69 Missing and 9 partials ⚠️
rust/lance/src/index/coverage.rs	68.96%	4 Missing and 5 partials ⚠️
rust/lance/src/dataset/scanner.rs	50.00%	0 Missing and 1 partial ⚠️

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