replay: delay per complete frame instead of per chunk

ReplayPacer (MsgListener+MsgTalker) sits between packetizer output and
downstream listeners; sleeps after each complete NMEA/UBX frame before
forwarding. FileBackend no longer owns the delay — it reads as fast as
possible and the pacer provides back-pressure on the same thread.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

msg/replay_pacer.py

@@ -0,0 +1,22 @@
+import time
+from msg.listener import MsgListener
+from msg.talker   import MsgTalker
+from msg.container import MsgContainer
+
+
+class ReplayPacer(MsgListener, MsgTalker):
+    """Sits between a packetizer and its listeners; sleeps after each
+    complete frame so file replay runs at a controlled rate."""
+
+    def __init__(self, delay: float = 1.0):
+        MsgListener.__init__(self)
+        MsgTalker.__init__(self)
+        self.delay = delay
+
+    def is_msg(self, msg: MsgContainer) -> bool:
+        return True
+
+    def on_recv(self, msg: MsgContainer):
+        if self.delay > 0:
+            time.sleep(self.delay)
+        self.call_listener(msg)