Warn when a behavior is re-entered while delivering a value

rxjs delivers a nested emission to every subscriber and then resumes
delivering the outer, older value to the remaining subscribers, which
leaves them permanently out of sync. Log the first occurrence per
behavior with a stack trace so the re-entrant path can be identified
from a rageshake.

src/state/ObservableScope.test.ts

@@ -237,3 +237,30 @@ describe("Reconcile", () => {
     expect(setup).toHaveBeenCalledWith(1);
   });
 });
+
+describe("behavior", () => {
+  it("warns when a subscriber re-enters the behavior synchronously", () => {
+    const warn = vi.spyOn(logger, "warn").mockImplementation(() => {});
+    const scope = new ObservableScope();
+    const source$ = new Subject<number>();
+    const behavior$ = scope.behavior(source$, 0);
+    // A subscriber that reacts to the value 1 by synchronously emitting 2
+    behavior$.subscribe((v) => {
+      if (v === 1) source$.next(2);
+    });
+    const seen: number[] = [];
+    behavior$.subscribe((v) => seen.push(v));
+
+    source$.next(1);
+
+    expect(warn).toHaveBeenCalledWith(
+      expect.stringContaining("Behavior re-entered"),
+      expect.any(String),
+    );
+    // Documents the hazard the warning is about: the later subscriber ends up
+    // with the stale value 1 even though the behavior's value is 2.
+    expect(behavior$.value).toBe(2);
+    expect(seen.at(-1)).toBe(1);
+    scope.end();
+  });
+});