feat(client): share the overlay across bundled copies and report by source (#2368)

* feat(client): share the overlay across bundled copies and report by source

The overlay DOM and problem state now live in a window singleton, so a
second bundled copy of the module (e.g. the webpack-dev-server client
once it adopts this overlay) renders into the same iframe instead of
stacking a duplicate. Fields missing from a state created by an older
copy are filled in place, and the Trusted Types policy moves into the
shared state too — creating two policies with the same name throws
under an enforced CSP.

* test(overlay): prevent re-rendering when clearing a source that reported nothing

Author: bjohansebas

README.md

@@ -449,6 +449,19 @@ overlay.showProblems("errors", ["Something broke"]);
 overlay.clear();
 ```
 
+The overlay state is a per-page singleton: every bundled copy of the module
+renders into the same overlay. Multiple clients can report side by side by
+passing a `source` — each source keeps its own slot and the overlay shows the
+union, with errors from any source taking precedence over warnings:
+
+```js
+overlay.showProblems("errors", ["Something broke"], "my-client");
+// Drop only this client's problems; other sources stay on screen.
+overlay.clear("my-client");
+// Without a source, everything is dismissed (same as Esc / backdrop / ×).
+overlay.clear();
+```
+
 The building indicator is exposed the same way:
 
 ```js

client-src/index.js

@@ -356,7 +356,10 @@ function createReporter() {
       return false;
     }
 
-    overlay.clear();
+    // Clear only this client's problems and the runtime errors — other
+    // clients sharing the overlay keep theirs.
+    overlay.clear("");
+    overlay.clear("runtime");
     return true;
   };