Separate bus messages from pipeline frames

BusMessage was a mixin tacked onto DataFrame / SystemFrame so the bus
could reuse the frame priority machinery. That made every bus message
also a Frame, which is misleading — bus messages travel on the bus, not
through pipelines. If a worker actually needs to ship a frame, it wraps
it in BusFrameMessage.

BusMessage is now a plain dataclass base carrying source/target.
BusDataMessage and BusSystemMessage are empty subclasses that exist
only as priority markers. The bus router and the priority queue check
``isinstance(item, BusSystemMessage)`` directly instead of
``isinstance(item, SystemFrame)``.

The serializer test that round-tripped DataFrame.name (a non-init
field) is rewritten against a local _MessageWithNonInit(BusDataMessage)
subclass so the serializer's init=False path stays covered.

tests/test_serializers.py

@@ -5,6 +5,7 @@
 #
 
 import unittest
+from dataclasses import dataclass, field
 
 from pydantic import BaseModel
 
@@ -35,6 +36,11 @@ class _UserInfo(BaseModel):
     address: _Address | None = None
 
 
+@dataclass(kw_only=True)
+class _MessageWithNonInit(BusDataMessage):
+    tag: str = field(init=False, default="default")
+
+
 class TestJSONMessageSerializer(unittest.TestCase):
     def setUp(self):
         self.serializer = JSONMessageSerializer()
@@ -232,14 +238,13 @@ class TestJSONMessageSerializer(unittest.TestCase):
 
     def test_non_init_fields_preserved(self):
         """Non-init dataclass fields survive round-trip via setattr."""
-        msg = BusDataMessage(source="task_a", target="task_b")
-        # name is a non-init field on DataFrame
-        msg.name = "custom_name"
+        msg = _MessageWithNonInit(source="worker_a", target="worker_b")
+        msg.tag = "custom_tag"
 
         data = self.serializer.serialize(msg)
         restored = self.serializer.deserialize(data)
 
-        self.assertEqual(restored.name, "custom_name")
+        self.assertEqual(restored.tag, "custom_tag")
 
 
 if __name__ == "__main__":