feat(udp): optional per-endpoint ZSTD compression

[MatejFranceskin]

Jira: SW-2443

Adds optional per-UDP-endpoint ZSTD compression, controlled by two new endpoint options:

ZstdCompression = true
ZstdDictionary  = /path/to.dict   # optional trained dictionary

It lets an endpoint compress its traffic to save bandwidth on a constrained link (e.g. a telemetry radio).

Behavior

• Each outgoing datagram (a coalesced batch of framed MAVLink messages) is compressed into one standalone ZSTD frame at level 3, optionally with a trained dictionary.
• Time-critical msgids (HEARTBEAT, SYSTEM_TIME, PING, MISSION_CURRENT, COMMAND_LONG, COMMAND_ACK, TIMESYNC, CURRENT_EVENT_SEQUENCE, REQUEST_EVENT) and datagrams that don't shrink are sent uncompressed.
• The receiver auto-detects the 4-byte ZSTD magic per datagram and decompresses; plain MAVLink passes through unchanged (no negotiation; mixed streams decode fine).
• Both ends must enable ZstdCompression and, if used, use the same ZstdDictionary.

Design

Compression sits only at the UDP syscall layer (compress before sendto in flush_pending_msgs, decompress after recvfrom in _read_msg); the shared MAVLink framer/CRC path and UART/TCP endpoints are untouched. UDP datagram atomicity means one datagram == one frame. RX_BUF_MAX_SIZE is raised to 8 KiB so a decompressed coalesced batch fits. The ZSTD library types are kept out of headers (opaque void* in ZstdCodec).

Includes tools/train-zstd-dict.sh to train a dictionary from captured MAVLink traffic, a default dictionary installed to <datadir>/mavlink-router/, and a README "ZSTD Compression" section.

Testing

• Codec unit round-trip: 9/9 (round-trip, magic detection, skip-list, size-fallback, plain pass-through, corrupt-frame drop).
• Two-instance loopback: all messages round-trip; wire carries ZSTD frames (~50% smaller) plus a plain frame for skip-listed messages.
• Cross-built for arm64 and validated on target: two-instance loopback, 81/81 messages round-trip, ZSTD magic confirmed in captured payloads.