Smallest-scope realisation of spec §"Built-in schedules" Damped: a
ConvergenceOptions::alpha field plumbed through run_chain to a new
Gaussian::damp_natural helper applied inside TruncFactor and
MarginFactor's propagate. alpha=1.0 default keeps every existing
golden bit-equal; alpha<1.0 stabilises oscillating fixed-point loops
on hard graphs.
Defers Schedule trait integration, nat-param convergence switch,
oscillation auto-detect, Residual/OneShot, and Synergy/ScoreFactor —
each gets its own future plan.
