fix(asc): fix relative timestamp roundtrip for 3+ messages

ASCReader was treating all timestamps as cumulative offsets from
start_time, ignoring the timestamps_format value. When reading a file
written with timestamps_format="relative" (per-event deltas) and
relative_timestamp=False, the reader now accumulates deltas into
start_time instead of adding each delta independently.

Without this fix, a 3-message roundtrip would produce:
  msg3: 0.7 + 100.0 = 100.7  (wrong, expected 101.0)

Also strengthen test_write_relative_timestamp_roundtrip to use 3
messages, exposing the bug that was masked by the 2-message case.

Author: Ju-Daeng-E

can/io/asc.py

@@ -295,7 +295,11 @@ def __iter__(self) -> Generator[Message, None, None]:
             msg_kwargs: dict[str, float | bool | int] = {}
             try:
                 _timestamp, channel, rest_of_message = line.split(None, 2)
-                timestamp = float(_timestamp) + self.start_time
+                if self.timestamps_format == "relative" and not self.relative_timestamp:
+                    self.start_time += float(_timestamp)
+                    timestamp = self.start_time
+                else:
+                    timestamp = float(_timestamp) + self.start_time
                 msg_kwargs["timestamp"] = timestamp
                 if channel == "CANFD":
                     msg_kwargs["is_fd"] = True

test/logformats_test.py

@@ -707,7 +707,8 @@ def test_write_relative_timestamp_roundtrip(self):
         """Messages written with relative format round-trip to their original timestamps."""
         msgs = [
             can.Message(timestamp=100.0, arbitration_id=0x1, data=b"\x01"),
-            can.Message(timestamp=100.5, arbitration_id=0x2, data=b"\x02"),
+            can.Message(timestamp=100.3, arbitration_id=0x2, data=b"\x02"),
+            can.Message(timestamp=101.0, arbitration_id=0x3, data=b"\x03"),
         ]
 
         with can.ASCWriter(self.test_file_name, timestamps_format="relative") as writer:
@@ -719,7 +720,8 @@ def test_write_relative_timestamp_roundtrip(self):
 
         self.assertEqual(len(result), len(msgs))
         self.assertAlmostEqual(result[0].timestamp, 100.0, places=3)
-        self.assertAlmostEqual(result[1].timestamp, 100.5, places=3)
+        self.assertAlmostEqual(result[1].timestamp, 100.3, places=3)
+        self.assertAlmostEqual(result[2].timestamp, 101.0, places=3)
 
     def test_write_relative_timestamps_are_per_event_deltas(self):
         """With timestamps_format='relative', each written timestamp is a delta from the