Default to gated dwell-regret ranking

[cmdr-chara]

Summary

• change the default value-model mode from weighted to the existing gated_dwell_regret path
• use the existing gate to choose between weighted ranking and dwell/positive-action/negative-feedback scoring
• retain explicit weighted rollback and automatic weighted fallback for invalid/closed gate configuration
• add tests for negative-feedback dominance, sustained dwell, positive actions, and default routing

Scope

This does not claim to optimize surveys, retention, session outcomes, or unavailable long-term labels. It only changes how existing per-post prediction heads are combined; request-relative normalization and the gate are documented.

Verification

• independent audit approved gate, fallback, rollback, score math, tests, and docs
• rustfmt --edition 2024 --check on modified Rust files
• git diff --check / git show --check

When the gate opens, MPN scoring is intentionally disabled as in the existing dwell-regret path; its checked-in default is already false. The public home-mixer subtree has no build manifest, so Rust tests could not run locally.