Add armed plugin-originated file watches across VFS mutations

Register core.watch before its armed notification, coalesce later changes
without losing post-notification writes, and clean up on caller death.
Observe mounted layers and open file handles, including descriptor/stream
writes, truncation, both rename paths and deletion. Track cancelled
mutations through process idle and retain visible paths across renames.
Document the protocol, expose native core discovery, and exercise watches
through SDK callbacks and raw WASI filesystem operations.

Card: SAU-359

sdk/src/calls.rs

@@ -26,6 +26,11 @@ impl Call {
         self
     }
     /// 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.
     pub fn on_notify(mut self, f: impl FnMut(&str, Value) + 'static) -> Self {
         self.notify = Some(Box::new(f));
         self