feat: automated echo quality test with time-window analysis

New --echo-test <secs> flag sends a 440Hz tone through relay echo,
records the return, and analyzes quality in 5-second windows:
- Per-window: frames sent/received, loss %, SNR (dB), correlation
- Detects quality degradation over time (compares first vs second half)
- Reports jitter buffer stats (depth, lost, late packets)
- Diagnoses jitter buffer drift and packet loss accumulation

Also exposes jitter_stats() on CallDecoder for diagnostics.

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

crates/wzp-client/src/lib.rs

@@ -10,6 +10,7 @@
 pub mod audio_io;
 pub mod bench;
 pub mod call;
+pub mod echo_test;
 pub mod handshake;
 
 #[cfg(feature = "audio")]