fix: Correct DSC decoder phasing sequence handling, MMSI and position decoding

Strip ITU-R M.493 phasing symbols (120-126) after dot pattern sync before
decoding message content. Fix MMSI BCD digit trimming direction and correct
test symbol encodings for position and MMSI edge cases.

tests/test_dsc.py

@@ -313,8 +313,9 @@ class TestDSCDecoder:
     def test_decode_mmsi_with_leading_zeros(self, decoder):
         """Test MMSI decoding handles leading zeros."""
         # Coast station: 002320001
-        # 00-23-20-00-01 -> [0, 23, 20, 0, 1]
-        symbols = [0, 23, 20, 0, 1]
+        # Padded to 10 digits: 0002320001
+        # BCD pairs: 00-02-32-00-01 -> [0, 2, 32, 0, 1]
+        symbols = [0, 2, 32, 0, 1]
         result = decoder._decode_mmsi(symbols)
         assert result == '002320001'
 
@@ -328,14 +329,16 @@ class TestDSCDecoder:
         # Symbols > 99 should be treated as 0
         symbols = [100, 32, 12, 34, 56]
         result = decoder._decode_mmsi(symbols)
-        # First symbol becomes 00
-        assert result == '003212345'[-9:]
+        # First symbol (100) becomes 00, padded result "0032123456",
+        # trim leading pad digit -> "032123456"
+        assert result == '032123456'
 
     def test_decode_position_northeast(self, decoder):
         """Test position decoding for NE quadrant."""
         # Quadrant 10 = NE (lat+, lon+)
         # Position: 51°30'N, 0°10'E
-        symbols = [10, 51, 30, 0, 10, 0, 0, 0, 0, 0]
+        # lon_deg = symbols[3]*100 + symbols[4] = 0, lon_min = symbols[5] = 10
+        symbols = [10, 51, 30, 0, 0, 10, 0, 0, 0, 0]
         result = decoder._decode_position(symbols)
 
         assert result is not None

utils/dsc/decoder.py

@@ -264,6 +264,20 @@ class DSCDecoder:
                 symbol_value = self._bits_to_symbol(symbol_bits)
                 symbols.append(symbol_value)
 
+        # Strip phasing sequence (RX/DX symbols 120-126) from the
+        # start of the message. Per ITU-R M.493, after the dot pattern
+        # there are 7 phasing symbols before the format specifier.
+        msg_start = 0
+        for i, sym in enumerate(symbols):
+            if 120 <= sym <= 126:
+                msg_start = i + 1
+            else:
+                break
+        symbols = symbols[msg_start:]
+
+        if len(symbols) < 5:
+            return None
+
         # Look for EOS (End of Sequence) - symbol 127
         eos_found = False
         eos_index = -1
@@ -404,9 +418,10 @@ class DSCDecoder:
             digits.append(f'{sym:02d}')
 
         mmsi = ''.join(digits)
-        # MMSI is 9 digits, might need to trim leading zero
+        # MMSI is 9 digits - trim the leading digit from the 10-digit
+        # BCD result since the first symbol's high digit is always 0
         if len(mmsi) > 9:
-            mmsi = mmsi[-9:]
+            mmsi = mmsi[1:]
 
         return mmsi.zfill(9)