Reset reused watcher state before starting a new ATVM run

- update the watcher start helper to stop any stale watcher instance for the same requested parent build name and remove its old state directory before starting fresh
- document that reused parent build names must not inherit stale cancelled, posted, state.json, or subruns state from older runs
- update the watcher install and design docs so the controller workflow explicitly treats stale reused-build-name state as part of startup cleanup

atvm/watcher-service/README.md

@@ -49,6 +49,7 @@ Typical workflow:
 1. Launch the ATVM run.
 2. Start the watcher for that run.
 3. The watcher polls the run log, process state, and `cmcReporter` artifacts.
+   - before starting, the helper resets any prior watcher state for the same requested build name so stale cancellation or posted markers do not leak into a new run
 4. For non-categorized runs, when the run reaches a terminal state:
    - `COMPLETED` or `FAILED`
      - build the final ATVM status
@@ -105,6 +106,12 @@ That results in:
 - service instance:
   - `atvm-run-watcher@e2e-redhat9.6-ubuntu24.04-w2k25-fc.service`
 
+The helper also:
+
+- stops any stale watcher instance for that same requested build name
+- removes the old watcher state directory for that requested build name
+- starts the new watcher with a clean state root for the new run
+
 ## Cancel Example
 
 ```bash