watcher: a stop during the first scan sends no startup notification (closes #271)

Run sent the startup notification right after the first scan without
checking the context. RunOnce returns as usual when shutdown cuts the
scan short, so a stop during the first scan sent "has started and
completed its initial scan" to every endpoint as the process shut down.
Run now sends it only when the context was not cancelled.

The new test cancels the context before Run starts, so the scan never
sends its lookup; a stop part way through a scan follows the same path.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ trial run of the finished image: https://git.eeqj.de/sneak/dnswatcher/issues/149
 
 # Completed Steps
 
+- 2026-10-13: a stop before the first scan completes sends no startup
+  notification; a scan that ran to the end still sends it (closes #271).
 - 2026-10-13: a save syncs the state file to disk before renaming it into place,
   so a crash cannot leave it empty and stop the next start (closes #264).
 - 2026-10-13: a `PORT` that cannot be bound stops startup with an error naming