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>

backend/file_backend.py

@@ -1,5 +1,4 @@
 import re
-import time
 from threading import Thread
 
 from backend.a_backend import ABackend
@@ -15,20 +14,11 @@ def source_id_from_path(path: str) -> str | None:
 
 
 class FileBackend(ABackend):
-    def __init__(self, files: list[str], delay: float = 1.0):
-        self._delay = max(0.0, min(10.0, delay))
+    def __init__(self, files: list[str]):
         self._entries = [{'path': p, 'source_id': source_id_from_path(p), 'xcvr': None, 'file': None, 'thread': None}
                          for p in files]
         self._running = False
 
-    @property
-    def delay(self) -> float:
-        return self._delay
-
-    @delay.setter
-    def delay(self, value: float):
-        self._delay = max(0.0, min(10.0, value))
-
     def register_xcvr(self, xcvr):
         for e in self._entries:
             if e['source_id'] == xcvr.name.strip():
@@ -56,7 +46,7 @@ class FileBackend(ABackend):
         self._running = False
         for e in self._entries:
             if e['thread'] and e['thread'].is_alive():
-                e['thread'].join(timeout=self._delay + 1.0)
+                e['thread'].join(timeout=2.0)
 
     def join(self):
         for e in self._entries:
@@ -77,6 +67,4 @@ class FileBackend(ABackend):
             if not data:
                 break
             if xcvr:
-                xcvr.on_recv(MsgContainer(data, xcvr))
-            if self._delay > 0:
-                time.sleep(self._delay)
+                xcvr.on_recv(MsgContainer(data, xcvr))