refactor: adapt UR decoding to cUR state machine API

diff --git a/simulator/kruxsim/mocks/uUR.py b/simulator/kruxsim/mocks/uUR.py
index 9770964..e125b43 100644
--- a/simulator/kruxsim/mocks/uUR.py
+++ b/simulator/kruxsim/mocks/uUR.py
@@ -46,10 +46,54 @@ class UREncoder(_UREncoder):
         return super().next_part().upper()
 
 
+# Decoder states, mirroring uUR's DECODER_* constants (ur_decoder_state_t)
+DECODER_OK = 0
+DECODER_PROCESSING = 1
+DECODER_NO_RESULT = 2
+DECODER_ERR_INVALID_SCHEME = 16
+DECODER_ERR_INVALID_TYPE = 17
+DECODER_ERR_INVALID_PATH_LENGTH = 18
+DECODER_ERR_INVALID_SEQUENCE_COMPONENT = 19
+DECODER_ERR_INVALID_FRAGMENT = 20
+DECODER_ERR_INVALID_PART = 21
+DECODER_ERR_INVALID_CHECKSUM = 22
+DECODER_ERR_MEMORY = 23
+DECODER_ERR_NULL_POINTER = 24
+
+_TERMINAL_STATES = (DECODER_OK, DECODER_NO_RESULT, DECODER_ERR_INVALID_CHECKSUM)
+
+
 class URDecoder(_URDecoder):
     """uUR exposes expected_part_count and processed_parts_count as plain
-    int attributes (zero for single-part URs). Mirror that here so the same
-    qr.py logic works against both decoders."""
+    int attributes (zero for single-part URs), and reports progress through a
+    state machine instead of is_complete()/is_success(). Mirror that here so
+    the same qr.py logic works against both decoders.
+
+    The Python decoder swallows the reason a part was rejected, so transient
+    errors are all reported as DECODER_ERR_INVALID_PART."""
+
+    def __init__(self):
+        super().__init__()
+        self._state = DECODER_PROCESSING
+
+    @property
+    def state(self):
+        return self._state
+
+    def receive_part(self, part):
+        # Terminal states are permanent: the part is not processed
+        if self._state in _TERMINAL_STATES:
+            return self._state
+        received = super().receive_part(part)
+        if isinstance(self.result, Exception):
+            self._state = DECODER_ERR_INVALID_CHECKSUM
+        elif self.result is not None:
+            self._state = DECODER_OK
+        elif not received:
+            self._state = DECODER_ERR_INVALID_PART
+        else:
+            self._state = DECODER_PROCESSING
+        return self._state
 
     @property
     def expected_part_count(self):
diff --git a/src/krux/qr.py b/src/krux/qr.py
index 29d7b04..ba1b64c 100644
--- a/src/krux/qr.py
+++ b/src/krux/qr.py
@@ -177,12 +177,16 @@ class QRPartParser:
             self.total = total
             return index - 1
         elif self.format == FORMAT_UR:
-            if not self.decoder:
-                from uUR import URDecoder
+            from uUR import URDecoder, DECODER_NO_RESULT, DECODER_ERR_INVALID_CHECKSUM
 
+            if not self.decoder:
                 self.decoder = URDecoder()
             data = data.decode() if isinstance(data, bytes) else data
-            self.decoder.receive_part(data)
+            if self.decoder.receive_part(data) in (
+                DECODER_NO_RESULT,
+                DECODER_ERR_INVALID_CHECKSUM,
+            ):
+                raise ValueError("Failed to decode UR")
         elif self.format == FORMAT_BBQR:
             from .bbqr import parse_bbqr
 
@@ -195,7 +199,9 @@ class QRPartParser:
     def is_complete(self):
         """Returns a boolean indicating whether or not enough parts have been parsed"""
         if self.format == FORMAT_UR:
-            return self.decoder.is_complete()
+            from uUR import DECODER_OK
+
+            return self.decoder.state == DECODER_OK
         keys_check = (
             sum(range(1, self.total + 1))
             if self.format in (FORMAT_PMOFN, FORMAT_NONE)
diff --git a/tests/test_qr.py b/tests/test_qr.py
index 7572316..17f0cde 100644
--- a/tests/test_qr.py
+++ b/tests/test_qr.py
@@ -156,6 +156,29 @@ def test_parser(mocker, m5stickv, tdata):
             assert res == tdata.TEST_DATA_B58
 
 
+def test_parser_ur_decoder_states(mocker, m5stickv, tdata):
+    """Transient UR decoder errors are ignored, terminal ones abort the parsing"""
+
+    import uUR
+    from krux.qr import QRPartParser
+
+    first_part = tdata.TEST_PARTS_FORMAT_MULTIPART_UR[0]
+
+    # Transient errors are expected while scanning, parsing goes on
+    parser = QRPartParser()
+    mocker.patch.object(
+        uUR.URDecoder, "receive_part", return_value=uUR.DECODER_ERR_INVALID_PART
+    )
+    parser.parse(first_part)
+    assert not parser.is_complete()
+
+    for terminal_state in (uUR.DECODER_NO_RESULT, uUR.DECODER_ERR_INVALID_CHECKSUM):
+        parser = QRPartParser()
+        mocker.patch.object(uUR.URDecoder, "receive_part", return_value=terminal_state)
+        with pytest.raises(ValueError):
+            parser.parse(first_part)
+
+
 def test_to_qr_codes(mocker, m5stickv, tdata):
     from krux.qr import to_qr_codes, FORMAT_NONE, FORMAT_PMOFN, FORMAT_UR, FORMAT_BBQR
     from krux.display import Display