guestd: Report shutdown and terminal status

[t0gun]

Context

The bootstrap branch only sends the existing hello frame and then waits. Once workloads run, guestd needs to report terminal outcomes and shut the guest down cleanly.

Scope

• Reap the supervised workload.
• Distinguish normal exit, non-zero exit, signal termination, and guestd-internal fatal errors.
• Report terminal status over the control path.
• Preserve existing hello and fatal compatibility.
• Power off the guest cleanly after terminal status is handled.
• If the control protocol needs a new frame kind, coordinate the paired scaled change explicitly.

Acceptance Criteria

• Normal workload exit results in a clean guest shutdown.
• Failed workload exit reports useful terminal information.
• Guestd fatal errors still use the fatal path.
• No zombie workload remains before shutdown.
• CI remains green.

[t0gun]

Tracked by #9.