Parse each AOG UDP datagram on its own so short packets do not drop the next one

[gunicsba]

Summary

The AOG UDP receive loop kept unparsed bytes in a static buffer between datagrams, and didn't reset its read index after moving leftover bytes to the front. A packet shorter than 8 bytes waited in the buffer for the next datagram; AOG's section control request (PGN 241) is 7 bytes. The packet after it was then read from the wrong offset, logged as Unknown start of message and dropped.

Other problems in the same loop:

• the 2 start bytes were read with two unsequenced index++ in one expression;
• it never checked that a whole packet was in the buffer before passing it on;
• index was a std::uint8_t.

Changes

• New header-only aog_packet::parse_datagram() (include/aog_packet_parser.hpp). It passes every complete packet in one datagram to a callback, and reports whether it stopped on an unknown start or a truncated packet, and where.
• AgIO never splits a packet across datagrams, so each datagram is now parsed on its own and nothing carries over to the next one.
• handle_incoming_packets() and handle_address_detection() both use it. The log line now says how many bytes of the datagram were dropped.
• tools/test_aog_packet_parser.cpp covers the field case (a 7-byte PGN 241 followed by a PGN 229), several packets in one datagram, an unknown start after a valid packet, truncated packets and empty packets. It is registered like test_async_log.

Notes

• Conflicts with Cut log noise: duplicate UDP warnings, global-destination RTS, non-section clients #98, which edits the same "Unknown start" log line. Whichever lands second should keep Cut log noise: duplicate UDP warnings, global-destination RTS, non-section clients #98's rate limiting on top of parse_datagram().
• The CRC is still not checked. It was commented out before as well.

Testing

• test_aog_packet_parser passes (MSVC and clang).
• Release build passes on Windows (MSVC).

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