Keep file-watch observation from serializing unrelated IO

Scope open-registration and rename/remove ordering to overlapping paths;
let descriptor mutations and directory creation proceed without that lock.
Prove blocked opens/writes cannot stall unrelated namespaces, including a
queued rename, and retain post-rename descriptor notifications. Document
handler-owned watch lifetimes and narrow paths; test natural idle exit.

Card: SAU-359

sdk/tests/wasi.rs

@@ -108,6 +108,15 @@ fn protocol() {
         }
         p.finishes();
     }
+    // A handler-owned watch does not keep an idle process alive after return,
+    // even though the host never answers the watch and stdin remains open.
+    let mut p = start(&wasm, &dir);
+    p.request("watch_once", json!({"path":"/mailbox"}));
+    let watch = p.read();
+    assert_eq!(watch["method"], "core.watch");
+    p.send(json!({"jsonrpc":"2.0","method":"core.notify","params":{"call":watch["id"],"name":"armed","params":{}}}));
+    assert_eq!(p.read()["result"], Value::Null);
+    p.finishes();
     let mut p = start(&wasm, &dir);
     p.request("rpc.discover", json!({}));
     let d = p.read();