Circuits.UART.Framing implementation for DL/T 645-2007 streams.
Accumulates bytes, identifies 0xFE preambles and 0x68 start delimiters,
verifies the secondary delimiter 0x68, deduces the expected frame length from
the length field L, validates the modulo-256 checksum and 0x16 end delimiter,
and resynchronizes byte-by-byte in the presence of noise.