Replace ScoredCandidate.signal_snapshot Vec<(String,f64)> with
SmallVec<[(SignalKey,f64); 4]> where SignalKey is Static(&'static str)
| Owned(Arc<str>):
- Compile-time-constant labels (sort bases, relevance, co_engagement,
preference_affinity) -> Static: zero allocation, pointer-copy clone.
- Dynamic {signal}_boost/_penalty/_decay labels built once per query in a
RuleLabels hoist (was format! per-candidate-per-rule), shared by Arc.
Cohort rescore hoisted the same way.
- scored accumulator pre-sized to candidates.len().
- Owned Strings rebuilt only at the two response-assembly sites (<= limit).
Byte-identical output: full 1896-test lib suite green. Measured win
(cargo bench --bench ranking, committed-base vs working-tree):
score_200_hot 23.89->21.04us (-11.9%), score_200_trending
27.66->25.85us (-6.5%), score_200_full_pipeline 27.88->26.97us (-3.3%).
smallvec promoted from the lock to a direct dep (no new dependency
surface). perf-sweep doc updated; T2 (per-term DashMap collapse) next.
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