Fix #9: DTC/freeze-frame parsing (phantom codes, Mode 02, hex frame index)

- parse_dtcs CAN branch is now message-aware: each ECU reply '<svc> <count>
  <pairs>' has its header stripped per-message, instead of flattening all lines
  and stripping svc+count once. With multiple ECUs the old code ate the second
  header as a DTC pair -> phantom codes. Critically, it does NOT blind-scan for
  svc (0x43 is a legal DTC first byte: C03xx) — a numbered ISO-TP continuation
  is distinguished by its 'N:' frame-index prefix, not by value.
- _line_bytes strips hex frame indices A:-F: (ISO-TP index cycles 0-F), not just
  0-9, so consecutive frames past the 10th aren't dropped.
- read_freeze_frame sends the correct '020200' (svc 02, PID 02, frame 00) and
  skips SID+PID+frame (+3), fixing the off-by-one that mis-read the freeze DTC.
- tests/test_dtc_parse.py: single-frame, multi-ECU (no phantom), numbered
  multiframe with a real C03xx continuation, hex index, non-CAN legacy.

Closes #9

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016yT89n4zR4qbrySoSiEyZs

tests/test_dtc_parse.py

@@ -0,0 +1,63 @@
+"""DTC parsing correctness (CAN multi-ECU, ISO-TP frame indices, single-frame)."""
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from obdcore.link import parse_dtcs, _line_bytes, decode_dtc
+
+
+def test_single_frame_can():
+    # 43 02 <P0133> <P0420>
+    lines = ["43 02 01 33 04 20"]
+    assert parse_dtcs(lines, 0x43, True) == ["P0133", "P0420"]
+    print("  single-frame CAN: OK")
+
+
+def test_multi_ecu_no_phantom_codes():
+    # Two ECUs each reply "43 <count> ...". The old flatten-and-strip-once code
+    # ate the second header (43 01) as a DTC pair -> phantom code. Each header
+    # must be stripped per-message.
+    lines = ["43 01 01 33", "43 01 04 20"]
+    got = parse_dtcs(lines, 0x43, True)
+    assert got == ["P0133", "P0420"], got
+    assert not any(d.startswith("C0301") or "4301" in d for d in got)
+    print(f"  multi-ECU CAN, no phantom codes: {got}: OK")
+
+
+def test_iso_tp_numbered_frames_hex_index():
+    # Multiframe with numbered continuations, including hex indices A:..F:.
+    # First frame "0:" starts with 43 08; continuations carry raw pairs. A
+    # continuation starting with 0x43 (a real C03xx code) must NOT be mistaken
+    # for a new header because it is a numbered continuation.
+    lines = [
+        "0: 43 08 01 33 04 20",
+        "1: 05 67 07 E4 43 45",     # ...last pair 43 45 == C0345 (real code, starts 0x43)
+        "2: 09 96 00 00 00 00",
+    ]
+    got = parse_dtcs(lines, 0x43, True)
+    assert got == ["P0133", "P0420", "P0567", "P07E4", "C0345", "P0996"], got
+    print(f"  numbered multiframe + C03xx continuation not misread: {got}: OK")
+
+
+def test_hex_frame_index_stripped():
+    assert _line_bytes("A: 11 22") == [0x11, 0x22]      # hex index A: now stripped
+    assert _line_bytes("F:1122") == [0x11, 0x22]
+    print("  _line_bytes strips hex ISO-TP frame indices A:-F:: OK")
+
+
+def test_legacy_non_can_still_works():
+    # 43 header per line (non-CAN)
+    lines = ["43 01 33 00 00", "43 04 20 00 00"]
+    got = parse_dtcs(lines, 0x43, False)
+    assert "P0133" in got and "P0420" in got, got
+    print(f"  non-CAN legacy parse intact: {got}: OK")
+
+
+if __name__ == "__main__":
+    test_single_frame_can()
+    test_multi_ecu_no_phantom_codes()
+    test_iso_tp_numbered_frames_hex_index()
+    test_hex_frame_index_stripped()
+    test_legacy_non_can_still_works()
+    print("\nALL DTC PARSE TESTS PASS")