Composes Score over a candidate slice, sorts descending, truncates.
Pure — no IO. Liked-rank-higher and high-skip-ranks-last behaviors
are stable across RNG seeds (jitter band is smaller than LikeBoost
and SkipPenalty).
Implements spec §6 weighted-shuffle scoring without the
contextual_match_score term (sub-plan #3 adds it). Pure Go, no DB
dependency; injectable RNG for deterministic tests. Coverage 100%
on score.go via the boundary tests.