CB-603: make FakeHerdr.calls thread-safe

Background loops call the fake from their own scheduler threads while a
test polls called() from the test thread. The list was a plain
ArrayList, so a nudge landing mid-stream threw
ConcurrentModificationException out of called().

It surfaced while I was verifying CB-598, which nudges more often, but
the race is on main today and is unrelated to that change.

824 tests, BUILD SUCCESS.

bridged/src/test/java/dev/ltms/bridged/herdr/FakeHerdr.java

@@ -7,6 +7,7 @@ import java.util.ArrayList;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 /**
  * Recording fake {@link HerdrClient} for unit/acceptance tests. Returns canned frames
@@ -22,7 +23,13 @@ public final class FakeHerdr implements HerdrClient {
     public static final long WORKER_PID = 4242;
 
     private final ObjectMapper mapper = new ObjectMapper();
-    public final List<Call> calls = new ArrayList<>();
+    /**
+     * Thread-safe on purpose. Background loops — {@link dev.ltms.bridged.msg.ReplyPushLoop} and the
+     * lead heartbeat — call this fake from their own scheduler threads while a test polls
+     * {@link #called} from the test thread. A plain {@code ArrayList} threw
+     * {@code ConcurrentModificationException} out of {@code called()} when a nudge landed mid-stream.
+     */
+    public final List<Call> calls = new CopyOnWriteArrayList<>();
     private boolean healthy = true;
     private final List<String> extraWorkspaces = new ArrayList<>();
     private final List<String> extraAgents = new ArrayList<>();