Idle partition stall

A watermark that stops advancing because it is the minimum across all inputs and one input has gone silent. It is a signature streaming failure: nothing is broken, nothing errors, and output simply stops. The fixes are all forms of admitting the silence — an idleness timeout that lets the quiet source be excluded, or the source emitting periodic heartbeats — and knowing the shape is what turns a three-hour investigation into a five-minute one.