refactor: use Brand roles and one controlled trace clock

Retained record hosts now control FlowWaterfall's existing playhead without a second playback clock. Seek, scrub, play/pause and full-record/live-follow actions preserve host selection. The shared clock keeps the original10Hz React report cadence and real elapsed playback rate; externalnull during active replay stops replay without overwriting follow. Numeric natural end remains unchanged.

Run ID: run_d6a451eac952679e802d772b0104ffcb