feat(rpc): auth handshake, server-push broker, audit logging

- auth_handshake.rs: connection-init protocol (magic 0x01, token, ack)
- push.rs: PushBroker manages per-identity push connections with gc
- server.rs: ConnectionState, auth handshake on first bi-stream, pass
  identity_key/session_token to RequestContext per stream
- client.rs: session_token in RpcClientConfig, auto auth handshake on connect
- middleware.rs: log_rpc_call with SHA-256 redaction, hex_prefix helper
- lib.rs: export auth_handshake and push modules

crates/quicproquo-rpc/Cargo.toml

@@ -17,6 +17,7 @@ tower            = { workspace = true }
 tracing          = { workspace = true }
 thiserror        = { workspace = true }
 dashmap          = { workspace = true }
+sha2             = { workspace = true }
 
 [dev-dependencies]
 tokio = { workspace = true, features = ["test-util"] }