perf(index): norm-addend cache and slim top-k heap for FTS scoring

[BubbleCal]

Stacked on #7625; last of the series. (Re-opened as a new PR after #7626 was consolidated into #7466 and its branch deleted, which auto-closed the original #7627.)

What

Two hot-loop changes, both score-identical (only tie ordering among equal scores can shift with the slimmer heap tuple):

• Per-search norm cache (Lucene's norm cache): Scorer::doc_norm exposes the BM25 doc-length denominator addend, and quantized (V3) searches bake the 256 possible addends once per partition-search. The OR streaming loop (collect_window_scores), the OR drain/single-essential completion paths, and the bulk conjunction scoring pass then score with one byte-norm load plus a cached addend instead of recomputing k1*(1-b+b*dl/avgdl) per doc. The factored expressions are evaluated identically, so scores are bit-equal to the uncached path.
• Slim top-k heap: heap entries shrink to a Copy tuple; the per-candidate (term, freq) pairs go to an append-only side log (FreqLog) and only the final top-k materialize a Vec — no more per-insert allocation, and heap sifts move 24 bytes instead of a Vec-carrying tuple.

Measured vs #7625 (its base)

Per-branch-tip wheels, 1000 queries × 8 concurrent. OR and AND on the
200M-doc v3-256 index (warm, prewarmed); phrase on the 50M-doc v3-256
positions index (steady-state passes, no prewarm):

query	#7625	this PR
OR 3w k10	0.0343s / 230 qps	0.0250s / 316 qps (1.37×)
OR 3w k100	0.0629s / 127 qps	0.0470s / 169 qps (1.34×)
AND 3w k10	0.0459s / 172 qps	0.0447s / 177 qps (+3%)
AND 3w k100	0.0924s / 86 qps	0.0892s / 89 qps (+3.5%)
phrase 3w k10 @50m	0.2256s	0.2187s (+3%)

The OR win comes from the streaming loop: recomputing the BM25 denominator
per (clause × doc) was its largest single cost.

Verification

• A/B vs the previous stack tip: score_diff=0 across AND (bulk and classic) and phrase query sets; 34 tie-reorders among equal scores from the heap-tuple change, as expected.
• cargo test -p lance-index, clippy -D warnings, fmt --check clean.

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