fix(parser): correct S3 frame terminator — bare ETX, not DLE+ETX

parse_s3 had the S3 terminator logic inverted vs the real S3FrameParser
in framing.py. It was terminating on DLE+ETX and treating bare ETX as
payload, which caused every bare 0x03 to be swallowed — bundling multiple
real S3 frames into one giant body until a DLE+ETX sequence happened to
appear. Result: 583-byte POLL_RESPONSE 'frames' containing many real
frames concatenated, all showing BAD CHK.

Fix: mirror S3FrameParser exactly —
  - Bare ETX (0x03) = real frame terminator
  - DLE+ETX (0x10 0x03) = inner-frame literal data (A4/E5 sub-frames),
    appended to body and parsing continues

https://claude.ai/code/session_014NczSHUz9uTzCAf4cVASTJ

parsers/s3_parser.py

@@ -33,7 +33,7 @@ STX = 0x02
 ETX = 0x03
 ACK = 0x41
 
-__version__ = "0.2.3"
+__version__ = "0.2.4"
 
 
 @dataclass
@@ -186,7 +186,7 @@ def parse_s3(blob: bytes, trailer_len: int) -> List[Frame]:
 
     IDLE        = 0
     IN_FRAME    = 1
-    AFTER_DLE = 2
+    IN_FRAME_DLE = 2   # saw DLE inside frame — waiting for next byte
 
     state = IDLE
     body = bytearray()
@@ -206,22 +206,15 @@ def parse_s3(blob: bytes, trailer_len: int) -> List[Frame]:
                 state = IN_FRAME
                 i += 2
                 continue
+            # ACK bytes, boot strings, garbage — silently ignored
 
         elif state == IN_FRAME:
             if b == DLE:
-                state = AFTER_DLE
+                state = IN_FRAME_DLE
                 i += 1
                 continue
-            body.append(b)
-
-        else:  # AFTER_DLE
-            if b == DLE:
-                body.append(DLE)
-                state = IN_FRAME
-                i += 1
-                continue
-
             if b == ETX:
+                # Bare ETX = real S3 frame terminator (confirmed from S3FrameParser)
                 end_offset = i + 1
                 trailer_start = i + 1
                 trailer_end = trailer_start + trailer_len
@@ -259,13 +252,27 @@ def parse_s3(blob: bytes, trailer_len: int) -> List[Frame]:
                 state = IDLE
                 i = trailer_end
                 continue
+            body.append(b)
 
+        else:  # IN_FRAME_DLE
+            if b == DLE:
+                # DLE DLE → literal 0x10 in payload
+                body.append(DLE)
+                state = IN_FRAME
+                i += 1
+                continue
+            if b == ETX:
+                # DLE+ETX inside a frame = inner-frame terminator (A4/E5 sub-frames).
+                # Treat as literal data, NOT the outer frame end.
+                body.append(DLE)
+                body.append(ETX)
+                state = IN_FRAME
+                i += 1
+                continue
             # Unexpected DLE + byte → treat as literal data
             body.append(DLE)
             body.append(b)
             state = IN_FRAME
-            i += 1
-            continue
 
         i += 1