Capture set shared_data->signal to true for every packet while the
decoder set it to CheckSignal() for every decoded frame. With
SignalCheckPoints on and a camera showing its lost-signal picture,
Analyse sometimes read Capture's true between two decoded frames, so the
signal flipped back for one frame: the event was closed for signal loss
and a zero-frame "Signal: Reacquired" event opened, about once a minute.
Capture now writes true only when no decoder measures the signal: no
SignalCheckPoints, Decoding=None, or on-demand decoding with no viewer.
It still writes true for the first packet after connect(), as before, so
Analyse doesn't see a signal loss before the first decoded frame and the
first event keeps its cause. The capture-failure path still writes false.
fixes#5235
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
