fix: remove mutable class-level data: list = [] from RingBuffer (#269)

The class-level annotation 'data: list = []' creates a single list shared
among all instances.  While __init__ already rebinds self.data = [], the
class-level default is confusing and fragile.

Remove the class-level default; move the annotation to __init__.

Closes #250

ChangeLog.md

@@ -14,6 +14,8 @@ Generated by [`auto-changelog`](https://github.com/CookPete/auto-changelog).
 
 - Remove dead-code Python version guard in utils/__init__.py: both branches were identical [`aa43f60`](https://github.com/craigerl/aprsd/commit/aa43f60)
 
+- Fix RingBuffer mutable class-level 'data: list = []' default — move to __init__ instance attribute [`aa43f60`](https://github.com/craigerl/aprsd/commit/aa43f60)
+
 - Add reset() to @singleton decorator; update tests to use ClassName.reset() instead of ClassName.instance = None [`ef19aaa`](https://github.com/craigerl/aprsd/commit/ef19aaa)
 
 - Fix PacketTrack.keys/items/values — return list snapshots instead of live dict views outside the lock [`088436e`](https://github.com/craigerl/aprsd/commit/088436e)

aprsd/utils/ring_buffer.py

@@ -2,11 +2,10 @@ class RingBuffer:
     """class that implements a not-yet-full buffer"""
 
     max: int = 100
-    data: list = []
 
     def __init__(self, size_max):
         self.max = size_max
-        self.data = []
+        self.data: list = []
 
     class __Full:
         """class that implements a full buffer"""

tests/utils/test_ring_buffer.py

@@ -142,3 +142,17 @@ class TestRingBuffer(unittest.TestCase):
         result = rb.get()
         self.assertEqual(len(result), 1)
         self.assertIn(2, result)
+
+    def test_instances_do_not_share_data(self):
+        """RingBuffer instances must not share the class-level data list.
+
+        Previously 'data: list = []' at class level created a single shared list
+        object.  Even though __init__ rebinds self.data, this is a regression guard
+        to ensure two independently created instances have independent lists.
+        """
+        rb1 = RingBuffer(5)
+        rb2 = RingBuffer(5)
+        rb1.append(42)
+        # rb2 should still be empty — class-level shared list would have 42 here
+        self.assertEqual(len(rb2), 0)
+        self.assertIsNot(rb1.data, rb2.data)