fix(recording): stop wf-recorder start-race producing 0.5s fragment filesHEADmain

On Wayland the video start path signalled any running wf-recorder with pkill -INT and then waited a fixed 0.1s before launching a new one. Releasing the compositor capture takes longer than that, so the new wf-recorder launched into a still-held grab, died immediately, and ffmpeg finalized a ~0.5-second fragment .mkv. Seeing nothing record, I'd press the key again and burst out a pile of fragments until the capture finally freed.

I replaced the fixed wait with a poll that waits until no wf-recorder remains (capped at 2s), the same move the stop path already made when it swapped its own fixed wait for cj/recording--wait-for-exit. Two supporting changes came with it. wf-recorder stderr no longer goes to /dev/null, so a failed grab shows up in the process buffer. And the sentinel now tells a ~0.5s failed start from a real stop, saying "failed to start, try again" instead of silently clearing, so one failure doesn't snowball into a blind-retry burst.

The poll and the fail-fast timing are unit-tested. The live capture needs a manual check.

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