Add message-name constants and reliable-channel helpers for netplay
session state transfer:
- Message-name constants: netplay_state, netplay_state_ready,
resync_state, resync_ready
- Chunked message codec (encode_message_chunks / decode_message /
decode_packets) reusing the existing 16 KiB reliable-channel
transport; send_message/process_reliable_messages refactored onto it
- send_netplay_state / receive_netplay_state for full session state
- send_ready / wait_for_ready ack helpers (10s timeout, same pattern
as receive_message); receive_message extracted to wait_for_message
with preserve of input_delay/disconnect side effects
- Large payloads (>1MB) accumulate into incoming_message until a full
postcard message parses
- Unit tests: 5MB payload round-trip through the chunked codec, ack
send/receive within timeout, missing ack times out with the standard
panic pattern
Closes#1
