perf(fts): accelerate wide and queries with bulk intersection

[BubbleCal]

Wide root Match AND queries still spend substantial time in per-document iterator advancement. This PR enables the existing bulk scoring route for four or more modern block-256, non-grouped clauses, and adds bounded pairwise SIMD/scalar DocID intersection before the competitive floor becomes positive. Posting offsets are recovered only for final survivors; positive-floor scoring retains the existing bulk implementation.

Ordinary root Match AND queries directly use Wand.search. Compound WandCursor continues to use the classic path covered by #9033. The separate Boolean MUST-child floor-propagation change in #9015 is not included.

Measurements

The comparison is incremental, from 85ec700bb4aacbc4e87f826f93fe7bc78fe53e41 to measured candidate 02c5f5a73fb29a47a0ff33293936e38c8f90c470. The current stacked tip, 0181df1136be876a04b327f24a0f1a4709263b58, has identical production code; subsequent changes only deduplicate tests and exclude benchmark files. The parent stack layer likewise retains the measured baseline's production code. These measurements establish the final bulk-routing/intersection increment, not the combined stack's speedup versus main.

Scenario / metric	Baseline	This PR	Benefit
4 terms, 0 matches / latency	1.237869 ms	0.335107 ms	3.694x speedup
5 terms, 5 matches / latency	1.552503 ms	0.448852 ms	3.459x speedup
8 terms, 0 matches / latency	3.984348 ms	0.841667 ms	4.734x speedup
16 terms, 5 matches / latency	8.309449 ms	1.882632 ms	4.414x speedup
4 terms, contiguous 32,768 matches / latency	2.627815 ms	1.309050 ms	2.007x speedup
16 terms, contiguous 32,768 matches / latency	13.304164 ms	3.344204 ms	3.978x speedup
16 terms, sparse correlated 32,768 matches / latency	13.401886 ms	3.383210 ms	3.961x speedup

Lower latency is better; benefit is baseline/current. Same AWS c7i.8xlarge, Xeon Platinum 8488C, Ubuntu 24.04, Rust 1.97.0/clang 18, repository release-with-debug with thin LTO and configured Haswell/AVX2 target. Both versions use Auto mode, CPU 2, one query in flight, LANCE_CPU_THREADS=1, and RAYON_NUM_THREADS=1.

The complete comparison contains 42 frozen cases: 65,536-document scaling/additional-distribution corpora and a 131,072-document control corpus. All use k=10, V3/block256, no positions/filter, and fully resident prewarm. High-hit scaling holds the intersection and each term's DF at 32,768 across term counts. Four independent processes per version/case run in ABBAABBA order, each with 2 s warmup, 5 s measurement, and 30 samples. Values are the median of four run medians. Independent scoring oracles and exact row/score-bit signatures passed.

Eight of 15 short-query controls show observed median regressions of approximately 0.6–2.2%. Some baseline runs have wider between-process spread; these are descriptive repeated measurements, not significance claims. Results cover warm index-query CPU work, not a 2.86B-row production or end-to-end dataset workload. Combined performance versus main remains unverified. Benchmark drivers and raw evidence are archived separately; no benchmark files are included in this stack.

Validation

• The equivalent combined implementation passed 762 inverted-index tests on ARM and x86.
• Six dataset integration cases passed: N4/N16 × 0/5/high matches, two fragments, V3/block256, independent BM25 row/score-bit oracle, verified MatchQuery plans, and a strict 10th/11th score gap.
• Thirteen existing WAND/certificate/tie tests passed, including bounded completion and exact overflow replay.
• Per-layer inverted-index suite: 762 passed; required all-workspace Clippy and formatting passed.
• Dataset integration rerun: 6 root Match AND cases and 13 WAND/certificate/tie cases passed. CI: pending.

Stack, 4/4: #9031 → #9032 → #9033 → this PR. Base: #9033.

Merge in stack order; retarget/restack the remaining PRs after their parent merges.

[lance-gatekeeper]

⚠️ Gate recommendation: approve with a non-blocking risk.

After the restack, this PR is correctly scoped to wide modern conjunction routing and pairwise block intersection while preserving exact membership, phrase and visibility handling, score bits, and dataset-level behavior. The remaining risk is performance reproducibility: the reported incremental baseline is production-equivalent to the current base, but the benchmark driver and raw evidence are absent from this tree, and 8 of 15 short-query controls show roughly 0.6–2.2% median regressions. LANCE_FTS_BULK_AND=off provides a runtime rollback if production latency buckets regress. No further change is requested for this pull request.