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/cancel-atvm-run-watcher.sh

@@ -1,6 +1,38 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+python3_write_cancelled_state() {
+  local state_file="$1"
+  python3 - "$state_file" <<'PY'
+import json
+import sys
+from datetime import datetime, timezone
+from pathlib import Path
+
+state_path = Path(sys.argv[1])
+state = {}
+if state_path.exists():
+    try:
+        state = json.loads(state_path.read_text(encoding="utf-8"))
+    except json.JSONDecodeError:
+        state = {}
+
+now = datetime.now(timezone.utc).isoformat()
+state["last_state"] = "CANCELLED"
+state["last_seen_at"] = now
+state["closed_at"] = now
+
+notes = state.get("notes", [])
+if not isinstance(notes, list):
+    notes = [str(notes)]
+if "Cancellation marker detected." not in notes:
+    notes.append("Cancellation marker detected.")
+state["notes"] = notes
+
+state_path.write_text(json.dumps(state, indent=2, sort_keys=True), encoding="utf-8")
+PY
+}
+
 usage() {
   cat <<'EOF'
 Usage:
@@ -29,4 +61,5 @@ fi
 RUN_DIR="${STATE_ROOT}/${BUILD_NAME}"
 mkdir -p "$RUN_DIR"
 touch "${RUN_DIR}/cancelled.marker"
+python3_write_cancelled_state "${RUN_DIR}/state.json"
 systemctl stop "atvm-run-watcher@${BUILD_NAME}.service" || true