Add frame header validation to prevent the parser from getting stuck
in an overflow loop when it loses sync with the UART byte stream
(e.g. after module restart or UART noise). This is the same latent
bug fixed in ld2450 (PR #14135) and present in ld2420.
The fix validates the first 4 bytes of each frame match a known
header (DATA or CMD) before accumulating data. On header byte
mismatch, the buffer resets and checks if the mismatched byte starts
a new frame. Also adds a return after buffer overflow reset to
prevent immediately re-accumulating the overflowed byte.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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