SESSION_RESET (41 03) is sent before each POLL frame to wake monitoring
units. The ETX lookahead in parse_bw only checked for ACK+STX directly
after ETX, so when 41 03 followed a frame's ETX, the check failed and the
ETX was swallowed into the body as a payload byte — giving a 19-byte body
instead of 17 for POLL frames and failing checksum validation.
Fix: scan past any SESSION_RESET (41 03) sequences when looking for the
next frame start, so the real ACK+STX boundary is found correctly.
Also adds SUM8 checksum validation to parse_s3, which previously left
checksum_valid=None for all S3 frames.
https://claude.ai/code/session_014NczSHUz9uTzCAf4cVASTJ
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