fix: bind BBQr parts to the first part of the stream

diff --git a/src/krux/bbqr.py b/src/krux/bbqr.py
index 71918ca..6f9fb7b 100644
--- a/src/krux/bbqr.py
+++ b/src/krux/bbqr.py
@@ -35,6 +35,15 @@ KNOWN_FILETYPES = {"P", "T", "J", "U"}
 
 BBQR_ALWAYS_COMPRESS_THRESHOLD = 5000  # bytes
 
+# Upper bound for the accumulated payload of an animated BBQr, in chars.
+# The header encodes the part total as 2 base36 chars, so a crafted stream may
+# announce up to 1295 parts and keep the parser accumulating them. A part count
+# limit can't be used here: small screens generate many small parts, so a few KB
+# PSBT already takes more than a hundred of them. Bound the total instead, at the
+# base32 expansion (8/5) of the 100 KB decompression limit. Anything above it
+# could not be decoded anyway.
+BBQR_MAX_PAYLOAD_LEN = 160 * 1024
+
 
 class BBQrCode:
     """A BBQr code, containing the data, encoding, and file type"""
diff --git a/src/krux/qr.py b/src/krux/qr.py
index ba1b64c..02a8133 100644
--- a/src/krux/qr.py
+++ b/src/krux/qr.py
@@ -131,6 +131,7 @@ class QRPartParser:
 
     def __init__(self):
         self.parts = {}
+        self.payload_len = 0
         self.total = -1
         self.format = None
         self.decoder = None
@@ -188,9 +189,23 @@ class QRPartParser:
             ):
                 raise ValueError("Failed to decode UR")
         elif self.format == FORMAT_BBQR:
-            from .bbqr import parse_bbqr
+            from .bbqr import parse_bbqr, BBQR_MAX_PAYLOAD_LEN
 
             part, index, total = parse_bbqr(data)
+            # Only the first part is passed to detect_format, and its encoding and
+            # file type are used to decode all of them. Parts of a BBQr aren't bound
+            # to each other by any checksum, so reject the ones that disagree with
+            # the first instead of splicing different streams into a corrupt result.
+            if data[2] != self.bbqr.encoding or data[3] != self.bbqr.file_type:
+                raise ValueError("BBQr header mismatch")
+            if self.total not in (-1, total):
+                raise ValueError("BBQr part total mismatch")
+            if self.parts.get(index, part) != part:
+                raise ValueError("Conflicting BBQr part")
+            if index not in self.parts:
+                self.payload_len += len(part)
+                if self.payload_len > BBQR_MAX_PAYLOAD_LEN:
+                    raise ValueError("BBQr payload too big")
             self.parts[index] = part
             self.total = total
             return index
diff --git a/tests/test_qr.py b/tests/test_qr.py
index ccff2fa..7371f31 100644
--- a/tests/test_qr.py
+++ b/tests/test_qr.py
@@ -279,6 +279,67 @@ def test_parse_pmofn_rejects_invalid_index(m5stickv):
         parse_pmofn_qr_part("p4of3 data")
 
 
+def test_parser_rejects_bbqr_header_mismatch(m5stickv):
+    """Parts must agree with the encoding and file type of the first part,
+    which is the one detect_format used to set up decoding"""
+    from krux.qr import QRPartParser
+
+    parser = QRPartParser()
+    parser.parse("B$HP0200414243")
+
+    with pytest.raises(ValueError, match="BBQr header mismatch"):
+        parser.parse("B$ZU0201444546")
+
+    assert parser.parts == {0: "414243"}
+
+
+def test_parser_rejects_bbqr_total_mismatch(m5stickv):
+    """A part announcing a different total belongs to another stream"""
+    from krux.qr import QRPartParser
+
+    parser = QRPartParser()
+    parser.parse("B$2P0300AAAAAAAA")
+
+    with pytest.raises(ValueError, match="BBQr part total mismatch"):
+        parser.parse("B$2P0100MZXW6YTB")
+
+    assert parser.total == 3
+    assert not parser.is_complete()
+
+
+def test_parser_rejects_conflicting_bbqr_part(m5stickv):
+    """The same index can be re-scanned, but not with different content"""
+    from krux.qr import QRPartParser
+
+    parser = QRPartParser()
+    parser.parse("B$2P0200AAAAAAAA")
+    parser.parse("B$2P0200AAAAAAAA")  # redundant scan of the same part is fine
+
+    with pytest.raises(ValueError, match="Conflicting BBQr part"):
+        parser.parse("B$2P0200MZXW6YTB")
+
+    assert parser.parts == {0: "AAAAAAAA"}
+
+
+def test_parser_rejects_oversized_bbqr_payload(m5stickv):
+    """Accumulated payload is bounded, a stream above it could not be decoded"""
+    from krux.qr import QRPartParser
+    from krux.bbqr import BBQR_MAX_PAYLOAD_LEN, int2base36
+
+    parser = QRPartParser()
+    part_size = 8192
+    parts = BBQR_MAX_PAYLOAD_LEN // part_size + 1
+    for index in range(parts - 1):
+        parser.parse(
+            "B$2P%s%s%s" % (int2base36(parts), int2base36(index), "A" * part_size)
+        )
+
+    with pytest.raises(ValueError, match="BBQr payload too big"):
+        parser.parse(
+            "B$2P%s%s%s" % (int2base36(parts), int2base36(parts - 1), "A" * part_size)
+        )
+
+
 def test_detect_format_propagates_base_exceptions(mocker, m5stickv):
     """detect_format catches genuine parsing errors (returns FORMAT_NONE) but
     must NOT swallow BaseException-level signals like KeyboardInterrupt.