feat: Phase 2 — relay daemon and client library with integration pipelines

wzp-relay:
- RelayPipeline: ingest → FEC decode → jitter buffer → FEC encode → send
- SessionManager: tracks active calls, idle expiry
- RelayConfig: TOML-based configuration
- Binary: accepts QUIC connections, receives media packets

wzp-client:
- CallEncoder: mic PCM → Opus encode → FEC → MediaPackets
- CallDecoder: MediaPackets → FEC decode → jitter → Opus decode → PCM
- CLI binary: connects to relay, sends test silence frames

99 tests passing across all 7 crates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/src/config.rs

@@ -0,0 +1,35 @@
+//! Relay daemon configuration.
+
+use serde::{Deserialize, Serialize};
+use std::net::SocketAddr;
+
+/// Configuration for the relay daemon.
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct RelayConfig {
+    /// Address to listen on for incoming connections (client-facing).
+    pub listen_addr: SocketAddr,
+    /// Address of the remote relay (for the lossy inter-relay link).
+    /// If None, this relay is the destination-side relay.
+    pub remote_relay: Option<SocketAddr>,
+    /// Maximum concurrent sessions.
+    pub max_sessions: usize,
+    /// Jitter buffer target depth in packets.
+    pub jitter_target_depth: usize,
+    /// Jitter buffer maximum depth in packets.
+    pub jitter_max_depth: usize,
+    /// Logging level (trace, debug, info, warn, error).
+    pub log_level: String,
+}
+
+impl Default for RelayConfig {
+    fn default() -> Self {
+        Self {
+            listen_addr: "0.0.0.0:4433".parse().unwrap(),
+            remote_relay: None,
+            max_sessions: 100,
+            jitter_target_depth: 50,
+            jitter_max_depth: 250,
+            log_level: "info".to_string(),
+        }
+    }
+}