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/src/calls.rs

@@ -27,10 +27,16 @@ impl Call {
     }
     /// Receive progress notifications before the result arrives.
     ///
-    /// For `core.watch`, await this call in a background task for the process's
-    /// lifetime. Scan only after `armed`, and rescan on `changed`; events can
-    /// coalesce. The call has no cancellation API. Do not use endpoint connect
-    /// retries to wait for file publication.
+    /// For `core.watch`, poll the call future alongside work in the handler, then
+    /// drop it when the handler returns. Do not put it in `background()` just to
+    /// keep the watch alive: background tasks keep the process alive, while the
+    /// watch only ends when the process ends. Dropping the future does not cancel
+    /// the host watch, but it lets an otherwise idle process exit normally.
+    ///
+    /// Scan only after `armed`, and rescan on `changed`. Events may coalesce, but
+    /// are not rate-limited: watch the narrowest useful path to avoid rescanning
+    /// for unrelated writes (for example, watch a mailbox, not all session data).
+    /// Do not use endpoint connect retries to wait for file publication.
     pub fn on_notify(mut self, f: impl FnMut(&str, Value) + 'static) -> Self {
         self.notify = Some(Box::new(f));
         self