Remove fixture races exposed by concurrent file IO

Read only atomically published journal batches, not in-progress spool tempfiles.
Wait for a guest-ready notification before closing stdin in the EOF test,
rather than treating process admission as handler readiness.

Card: SAU-359

cli/tests/plugins/probe/src/main.rs

@@ -83,6 +83,9 @@ fn unknown_id() {
 }
 fn wait_eof(lines: &mut Lines) {
     std::fs::write("/state/waiting", "yes").unwrap();
+    send(json!({"jsonrpc":"2.0","method":"core.notify","params":{
+        "name":"waiting-eof", "params":{"pid":std::env::var("SAUSAGE_PID").unwrap()}
+    }}));
     for _ in lines {}
     std::fs::write("/state/stopped", "yes").unwrap();
 }