Adds a Live/DVR panel (stream type, window classification, live gap,
DVR window length, buffer-ahead, presentation anchor + wall-clock) and
seek controls (seek-to-live, -10/-30s, jump to window start) to the SPF
segment-loading harness. All derived sandbox-side from presentation
streamType + the native video; degrades cleanly to on-demand.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>