Record cancelled watcher state on ATVM run cancellation

- update the watcher cancel helper so it writes a final CANCELLED state into state.json before stopping the service
- record cancellation timestamps and a cancellation note in the watcher state file for clearer post-run inspection
- update the watcher service docs so the documented cancel behavior matches the state-file handling

atvm/watcher-service/INSTALL.md

@@ -173,6 +173,13 @@ Cancel example:
   --build-name e2e-redhat9.6-ubuntu24.04-w2k25-fc
 ```
 
+The cancel helper should:
+
+- write `cancelled.marker`
+- update `state.json` so the final watcher state is `CANCELLED`
+- stop the watcher instance
+- avoid any Mattermost post for that run
+
 ## Operational Notes
 
 - This is not a daemon.
@@ -194,6 +201,7 @@ Expected terminal behavior:
   - verify `ok`
   - exit
 - `CANCELLED`
+  - write final `CANCELLED` state to `state.json`
   - do not post
   - exit
 - `TERMINATED`