feat(signals): add WritableResultExt trait (#5338)

* feat(signals): add WritableResultExt trait

Add WritableResultExt mirroring the existing ReadableResultExt,
providing mutable access to signal values holding Result<T, E>.

Methods:
- as_mut(): returns Result<WritableRef<T>, WritableRef<E>>
- unwrap_mut(): returns WritableRef<T>, panics if Err

Closes #5293

* refactor(signals): use descriptive expect messages in WritableResultExt::as_mut

Replace bare .unwrap() calls with .expect() containing a descriptive
message about the failure mode. This gives better diagnostics if the
Result variant changes between the read and write lock acquisitions.

* switch to a single write to avoid race conditions with sync signals

---------

Co-authored-by: Evan Almloff <evanalmloff@gmail.com>

packages/signals/src/read.rs

@@ -419,9 +419,19 @@ pub trait ReadableResultExt<T, E>: Readable<Target = Result<T, E>> {
         T: 'static,
         E: 'static,
     {
-        <Self::Storage as AnyStorage>::try_map(self.read(), |v| v.as_ref().ok()).ok_or(
-            <Self::Storage as AnyStorage>::map(self.read(), |v| v.as_ref().err().unwrap()),
-        )
+        let read = self.read();
+        match read.deref() {
+            Ok(_) => Ok(<Self::Storage as AnyStorage>::map(read, |v| {
+                v.as_ref()
+                    .ok()
+                    .expect("Result variant changed between read and map")
+            })),
+            Err(_) => Err(<Self::Storage as AnyStorage>::map(read, |v| {
+                v.as_ref()
+                    .err()
+                    .expect("Result variant changed between read and map")
+            })),
+        }
     }
 }