perf(fts): bound classic conjunction scoring in dense windows

[BubbleCal]

Classic 4/5-term AND evaluation can decode and score every follower even when a partial exact score already proves the candidate cannot compete. Applying score-first evaluation universally is costly when posting overlap is low. This PR enables it only inside a metadata-certified dense range.

• Prove continuous coverage from each current posting block's first/last/count metadata, while retaining actual follower membership checks.
• Build conservative suffix bounds only for eligible live windows and lazily cache exact BM25 norm addends.
• Preserve canonical query-order f32 scores and inclusive tie behavior.
• Reuse an unbounded zero-floor window and invalidate it when local, shared, or compound floors become positive; clear eligibility before phrase evaluation.

The classic path remains used by compound WandCursor and explicit classic searches after #9030 enables standalone wide-query bulk routing.

Most of this diff is regression coverage: approximately 327 production-section additions and 1,334 test additions. Shared fixtures retain the distinct dense/sparse, malformed-metadata, short-tail, high-DocID, score-bit, tie, norm-fallback, seek/shallow, and floor-transition cases. No isolated main A/B result is claimed for this layer.

Validation

• Per-layer inverted-index suite: 603 passed.
• Required all-workspace Clippy and formatting: passed.
• PR metadata/gatekeeper checks: pending. Full hosted test workflows only run for main/release/** bases; this intermediate layer has been tested and linted locally. The full hosted matrix will run after its base is retargeted to main following the parent merge.

Stack, 3/4: #9031 → #9032 → this PR → #9030. Base: #9032. Uses the lazy posting-cursor support introduced in #9031.

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

[lance-gatekeeper]

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

The score-first path preserves canonical score bits and floor semantics in the focused regression set, and its strict eligibility keeps unsupported scorers and formats on the existing path.

The remaining risk is performance rather than correctness: this layer adds dense-window certification and a lazy norm cache without an isolated latency/CPU comparison, so its net benefit—and possible short-tail overhead—remains unverified. Keep the scope narrow until representative classic 4/5-term workloads confirm the trade-off; if the incremental gain is small, progressive scoring after full DocID alignment is the simpler alternative.

This PR is stacked on #9032, which depends on #9031; the safe landing order remains #9031 → #9032 → this PR.