fix: harden DSC decoder against noise-induced false decodes

Stricter dot pattern detection (200 bits/100 alternations), bounded
phasing strip (max 7), symbol check bit parity validation, EOS minimum
position check, strict MMSI decode (reject out-of-range symbols),
format-aware telecommand extraction, and expanded critical category
detection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

utils/dsc/constants.py

@@ -89,6 +89,15 @@ TELECOMMAND_CODES = {
     201: 'POLL_RESPONSE',      # Poll response
 }
 
+# Full 0-127 telecommand lookup (maps unknown codes to "UNKNOWN")
+TELECOMMAND_CODES_FULL = {i: TELECOMMAND_CODES.get(i, "UNKNOWN") for i in range(128)}
+
+# Format codes that carry telecommand fields
+TELECOMMAND_FORMATS = {112, 114, 116, 120, 123}
+
+# Minimum symbols (after phasing strip) before an EOS can be accepted
+MIN_SYMBOLS_FOR_FORMAT = 12
+
 
 # =============================================================================
 # DSC Symbol Definitions