fix(recording): scope wf-recorder stop signal to our own process

Stopping a Wayland recording ran pkill -INT wf-recorder, which signals every wf-recorder on the system — including an unrelated screen capture the user started outside Emacs. The stop path now scopes the producer-first interrupt to the wf-recorder child of our own recording shell via pkill -P <shell-pid>, in the new cj/recording--interrupt-child-wf-recorder helper.

The producer-first ordering is unchanged: wf-recorder still gets SIGINT before the process-group signal so ffmpeg sees a clean EOF on pipe:0 and finalizes the MKV. The orphan-cleanup at recording start stays a broad by-name kill on purpose — those leftover recorders come from crashed sessions whose shells are already dead, so there is no live PID to scope to. Tests cover the scoped call, the nil-PID no-op, and that the bare system-wide form is never used.

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