StreamSourceSequenceReleasePreloaded

Client request: release a held pin onto the output. Mirrors SSS’s release model — SSQ does not autopilot; every transition is driven by an explicit release_preloaded.

Cold start (no active source): the held pin’s queue drains rebased (anchor_pts_us applied) and becomes the active source. Mid-stream (another source is currently emitting): the release is queued and applied at the current source’s next EOF.