perf(rowids): accelerate dense bitmap decoding

[jiaoew1991]

Summary

Add dense and near-dense bitmap decode paths.

This follows #8713 and targets RangeWithBitmap segments after the sequential cursor has removed repeated prefix scans.

The decoder now:

• emits a full 0xff byte as one contiguous eight-value range;
• expands full bytes with at least six set bits as contiguous runs;
• selects the specialized dense stream once for a single bitmap segment;
• seeds the cursor with the cardinality used for that decision, avoiding a second bitmap scan;
• keeps multi-segment sequences on the existing sparse path because one stream-wide decoder cannot safely assume that every segment has the same density.

The sparse cursor and bitmap loop remain separate and source-equivalent to main. This avoids the measurable fallback regression caused by performing adaptive dispatch in every batch.

Bitmap.data and Bitmap.len remain publicly accessible for source compatibility. A proposed popcount cache was removed because direct mutation of the public byte vector could otherwise make the cached cardinality stale. The on-disk encoding remains byte-for-byte unchanged.

Performance

Measured on Linux x86_64 with release-with-debug, 1,000,000 output rows, batch size 1,024, 10 Criterion samples, 1 second warm-up, and 3 seconds measurement. Both binaries used the same benchmark source. Baseline was current main at d57d0fb42; candidate was fbde7d600.

Shape	Payload	main	This PR	Change
50% density (holes_2)	no	2.243 ms	2.274 ms	+0.96% (within Criterion noise threshold)
50% density (holes_2)	yes	2.600 ms	2.594 ms	-0.22% (no significant change)
~94% density (holes_17)	no	2.484 ms	1.600 ms	-35.58%
~94% density (holes_17)	yes	2.592 ms	1.677 ms	-35.14%

Linux perf attributes the dense-shape improvement to the intended decoder change: on main, SegmentCursorState::extend_range accounts for 68.17% of CPU samples; this PR moves that work to SegmentCursorState::extend_dense_range (48.61% of samples) while reducing end-to-end time by 35.58%. For the 50%-density fallback, both main and this PR remain in SegmentCursorState::extend_range (66.83% and 70.13% respectively); no adaptive-dispatch helper appears in the hot path.

Validation

• cargo test -p lance-table --lib (365 passed)
• cargo clippy --all --tests --benches -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• full-width-byte regression coverage, including the previous shift-by-8 panic
• dense sequential cursor coverage across batch and byte boundaries, tail reads, past-end reads, and rewind
• sparse and multi-segment fallback coverage
• concurrent multi-batch stable row-ID coverage with deletions
• unsorted stable row-ID index coverage
• direct public-byte mutation cardinality coverage
• public-field source-compatibility coverage through U64Segment::RangeWithBitmap
• byte-exact serde coverage

Dependency #8713 is merged. This PR targets main directly and contains only the bitmap follow-up.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The latest merge from main leaves the reviewed bitmap patch unchanged relative to its new base, and the incoming changes do not touch the row-ID path. The focused crate suite still passes, so the previously verified dense-decoding speedup, sparse fallback, cursor behavior, public bitmap mutation, and byte-exact serialization remain sound.