perf(scanner): answer unfiltered count_rows from fragment metadata

[LuciferYang]

Summary

Scanner::count_rows always built and executed a count plan, even when the count could be satisfied from fragment metadata alone. For a plain count with no row-level filter or search this scanned row data unnecessarily — especially wasteful when the scanner is restricted to a subset of fragments via with_fragments (#6970).

This adds a metadata-only fast path to Scanner::count_rows: when nothing in the scan needs to inspect row data, it sums each fragment's live row count (physical rows − deletions, both tracked in fragment metadata) instead of building and executing a plan. Dataset::count_rows(None) already had such a fast path via count_all_rows; this brings the same benefit to the Scanner path (and, crucially, to fragment-restricted counts).

The fast path falls back to the existing count plan — preserving its results and its errors — whenever any of these is set:

• a row-level filter, vector / full-text search, index_segments, fast_search, or include_deleted_rows;
• order_by or limit / offset (the plan rejects these when combined with the count aggregate);
• a dynamic-only projection such as SELECT 1 (also rejected by the plan).

The shared per-fragment summing is factored into Dataset::count_fragment_rows, which count_all_rows now also uses. Its fan-out uses the module-standard io_parallelism() bound, matching the sibling count_deleted_rows. In the common (new-format) case the count is answered entirely from cached metadata with zero I/O; only legacy/uncached fragments fall back to per-fragment metadata reads.

Closes #6970.

Test plan

• New test_count_rows_metadata_only covering: whole dataset, fragment subset, empty fragment list, deletions (whole-dataset and subset), include_deleted_rows fallback, limit / offset / order_by error preservation, filtered-count fallback, and all-rows-deleted. It asserts zero read/write IOPS on the metadata fast path and > 0 reads on the plan fallback.
• cargo test -p lance --lib — count, scanner (142), and dataset_io (52) suites pass.
• cargo fmt --all --check clean.
• cargo clippy -p lance --all-targets -- -D warnings clean.

[claude]

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[codecov]

Codecov Report

❌ Patch coverage is 97.80220% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/scanner.rs	97.61%	0 Missing and 2 partials ⚠️

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