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/src/middleware.rs

@@ -1,11 +1,15 @@
-//! Tower-based middleware layers for the RPC server.
+//! Middleware layers for the RPC server.
 //!
-//! - `AuthLayer`: validates session tokens and attaches identity to context.
-//! - `RateLimitLayer`: per-IP request rate limiting.
+//! - `SessionValidator`: validates session tokens and resolves identity keys.
+//! - `RateLimiter`: per-key sliding-window rate limiting.
+//! - `log_rpc_call`: structured audit logging for RPC calls.
 
 use std::time::{Duration, Instant};
 
 use dashmap::DashMap;
+use sha2::Digest;
+
+use crate::error::RpcStatus;
 
 // ── Auth middleware ──────────────────────────────────────────────────────────
 
@@ -70,6 +74,45 @@ impl RateLimiter {
     }
 }
 
+// ── Audit logging ───────────────────────────────────────────────────────────
+
+/// Log an RPC call with timing and caller info.
+///
+/// When `redact` is true, the identity key is hashed before logging so that
+/// raw keys never appear in log output.
+pub fn log_rpc_call(
+    method_name: &str,
+    identity_key: Option<&[u8]>,
+    latency: Duration,
+    status: RpcStatus,
+    redact: bool,
+) {
+    let ik_display = match identity_key {
+        Some(ik) if redact => {
+            let hash_input_len = 8.min(ik.len());
+            let digest = sha2::Sha256::digest(&ik[..hash_input_len]);
+            format!("h:{}", hex_prefix(&digest))
+        }
+        Some(ik) => hex_prefix(ik),
+        None => "anonymous".to_string(),
+    };
+    tracing::info!(
+        method = method_name,
+        identity = %ik_display,
+        latency_ms = latency.as_millis() as u64,
+        status = ?status,
+        "rpc"
+    );
+}
+
+fn hex_prefix(bytes: &[u8]) -> String {
+    bytes
+        .iter()
+        .take(4)
+        .map(|b| format!("{b:02x}"))
+        .collect::<String>()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -93,4 +136,19 @@ mod tests {
         std::thread::sleep(Duration::from_millis(5));
         assert!(rl.check(key)); // window expired
     }
+
+    #[test]
+    fn hex_prefix_formats_first_4_bytes() {
+        assert_eq!(hex_prefix(&[0xab, 0xcd, 0xef, 0x01, 0x99]), "abcdef01");
+        assert_eq!(hex_prefix(&[0x00, 0xff]), "00ff");
+        assert_eq!(hex_prefix(&[]), "");
+    }
+
+    #[test]
+    fn log_rpc_call_does_not_panic() {
+        // Verify that audit log function does not panic with various inputs.
+        log_rpc_call("test.method", None, Duration::from_millis(42), RpcStatus::Ok, false);
+        log_rpc_call("test.method", Some(&[1, 2, 3, 4, 5, 6, 7, 8]), Duration::from_millis(1), RpcStatus::Internal, true);
+        log_rpc_call("test.method", Some(&[0xab]), Duration::ZERO, RpcStatus::Unauthorized, true);
+    }
 }