fix terminal command completion, exit status, and PTY session recovery

[3316891527]

Summary

• Add an explicit per-command completion marker so command execution does not depend solely on prompt parsing.
• Propagate the shell exit code through CommandExecutionEvent.
• Report dispatch failures and interactive-session misuse as completed failure events instead of leaving callers waiting.
• Avoid treating canonical PTY FIONREAD=0 as proof that a command is waiting for input.
• Serialize command events and fix the Process.exitValue() liveness contract.

Validation

• git diff --check passed.
• Android compilation was attempted with /root/android-sdk; the environment's aidl binary cannot start because the host lacks the required glibc loader.

This PR changes the terminal core only; it does not change the Linux rootfs or PRoot image.