Exit without stopping when there is nothing to stop

[cressie176]

Closes #22.

system.exit(code) called from the rejection of a failed start() announced a second, empty stop pair. The stop which follows a failed start had already reset the system, so the stop() inside exit ran a fresh stop which skipped every component and announced itself regardless.

Behaviour

• exit() on a system with nothing to stop exits at once and announces no stop. Nothing to stop means no start in flight, no stop in flight or left failed, and no component started: a system which never started, has already stopped, or whose start failed and the stop which followed has finished.
• A stop which failed leaves components started, so an exit after it still retries the stop and the code still reflects the outcome. A new scenario guards this so the check cannot skip too much.
• A bare stop() after a failed start is unchanged and still announces the empty pair, by the existing rule in system-events.md that an operation with nothing to do is announced like any other. The alternative, making stop() silent on a stopped system, would have overruled that rule and the two-pair assertion in "Stopping a system which has already stopped".

Found empirically

• The existing "announced X before exiting" step used includes, so it could not see a duplicate. The exiting library gains a step which compares the whole event trace the child program printed as a table.
• The child program's exit wrapper returned nothing, which only surfaced once a behaviour replaced start() and the program tried to chain on it.
• Dropping the start-in-flight term of the check fails "An exit while the system is starting", so each term of the guard is load-bearing except the stop-in-flight one, which only matters in a window no child-process scenario can reach without contortion; see the issue comment.

Tests

Four new child-process scenarios in test/features/exiting.md: three red before the fix with the duplicate pair visible in the diff, green after, and red again with the guard broken. 1357 steps pass, lint and typecheck clean.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (69ebff3) to head (6a24a70).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #23   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines          801       810    +9     
=========================================
+ Hits           801       810    +9     

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