Add NatsParser that reads NATS protocol commands from
ReadOnlySequence<byte>. Identifies commands by first 2 bytes using
case-insensitive bit masking. Handles PUB/HPUB payload reading with
stateful _awaitingPayload for split-packet scenarios. Uses Span<Range>
for zero-allocation argument splitting and ParseSize for ASCII decimal
parsing. Includes CommandType enum, ParsedCommand struct, and
ProtocolViolationException.
14 tests covering PING, PONG, CONNECT, INFO, SUB (with/without queue),
UNSUB (with/without max), PUB (with/without reply, zero payload),
HPUB, multiple commands, and case insensitivity.
