feat: add large BW→S3 write frame checksum validation in s3_parser

parsers/s3_parser.py

@@ -109,6 +109,28 @@ def _try_validate_sum8(body: bytes) -> Optional[Tuple[bytes, bytes, str]]:
     return None
 
 
+def _try_validate_sum8_large(body: bytes) -> Optional[Tuple[bytes, bytes, str]]:
+    """
+    Large BW->S3 write frame checksum (SUBs 68, 69, 71, 82, 1A with data).
+
+    Formula: (sum(b for b in payload[2:-1] if b != 0x10) + 0x10) & 0xFF
+    - Starts from byte [2], skipping CMD (0x10) and DLE (0x10) at [0][1]
+    - Skips all 0x10 bytes in the covered range
+    - Adds 0x10 as a constant offset
+    - body[-1] is the checksum byte
+
+    Confirmed across 20 frames from two independent captures (2026-03-12).
+    """
+    if len(body) < 3:
+        return None
+    payload = body[:-1]
+    chk = body[-1]
+    calc = (sum(b for b in payload[2:] if b != 0x10) + 0x10) & 0xFF
+    if calc == chk:
+        return payload, bytes([chk]), "SUM8_LARGE"
+    return None
+
+
 def _try_validate_crc16(body: bytes) -> Optional[Tuple[bytes, bytes, str]]:
     """
     body = payload + crc16(2 bytes)
@@ -137,11 +159,16 @@ def validate_bw_body_auto(body: bytes) -> Optional[Tuple[bytes, bytes, str]]:
     Try to interpret the tail of body as a checksum in several ways.
     Return (payload, checksum_bytes, checksum_type) if any match; else None.
     """
-    # Prefer SUM8 first (it fits small frames and is cheap)
+    # Prefer plain SUM8 first (small frames: POLL, read commands)
     hit = _try_validate_sum8(body)
     if hit:
         return hit
 
+    # Large BW->S3 write frames (SUBs 68, 69, 71, 82, 1A with data)
+    hit = _try_validate_sum8_large(body)
+    if hit:
+        return hit
+
     # Then CRC16 variants
     hit = _try_validate_crc16(body)
     if hit: