Two root causes for the waterfall/monitor lockup when scrolling past the
2.4 MHz RTL-SDR span:
1. safe_terminate() sent SIGKILL but never called wait(), leaving a
zombie process that kept the USB device handle open. The subsequent
capture restart failed the USB probe and the monitor could not use
the shared IQ path, falling back to a process-based monitor that
stole the SDR from the waterfall.
2. When the frontend created a new WebSocket after a failure, the old
handler's finally block called _set_shared_capture_state(running=False)
which could race with the new handler's running=True, making the
shared monitor path unavailable. Added a generation counter so only
the owning handler can clear the shared state.
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