Smooth cold-start eligibility thresholds

[cmdr-chara]

Summary

• replace binary impression/follower cold-start cliffs with bounded smoothstep confidence decay
• make decay widths configurable; width 0 restores exact legacy comparisons
• scale the single-candidate boost by combined confidence
• refresh cached engagement counts for authors inside the follower decay band
• preserve the existing random target-slot sampling for multi-slot experiments

Verification

• exact boundary, monotonicity, zero-width compatibility, partial boost, zero-at-band-end, and hydration regressions added
• independent audit caught and corrected an unrelated target-slot change before final approval
• rustfmt --edition 2024 --check on modified Rust files
• git diff --check / git show --check

Existing experiment/corpus arms, original-only eligibility, freshness, rank-position, slot constraints, random slot-range semantics, and one-candidate selection are preserved. The public home-mixer subtree has no build manifest, so tests could not run locally.