[LinuxBoy-96]

• Problem: start() waits for cosmic-comp's environment (env_rx.await) without listening for restart requests at the same time. The channel stays open forever: the task that reads the compositor's socket will never see its end, because cosmic-session itself keeps the other end of the socket (comp, kept alive in the spawned task).
• When: cosmic-comp exits before sending its environment: GPU init failure, panic at startup, startup with no output, or restart after a crash.
• Consequence: the session stays frozen on a black screen or a console, never restarts and never gives control back to the login manager.

Related: pop-os/cosmic-comp#3044 aborts a stuck cosmic-comp; this makes sure the session comes back even if the new one can't start.

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