fix(net): broadcast LEFT on disconnect, add keepalive/reaper, cap PLC hiss

Three reported bugs traced to one root cause plus two missing designed features:

1. Stale users + eternal PLC hiss (root cause): ConnSession::close() silently
   erased dropped users without broadcasting UserEvent::LEFT, so peers never
   learned the user left and their audio engines never called remove_stream —
   Opus PLC synthesized comfort noise forever. Fix: broadcast_left() helper
   + close() broadcasts LEFT before erasing.

2. PLC cap (defense-in-depth): on_playback now caps pure PLC at ~2s, then
   emits digital silence so a stale stream can never hiss forever even if
   remove_stream is skipped. Resets automatically on fresh packets.

3. No timeout / no ping: client never sent Ping, server had no last_seen /
   reaper, so half-open connections (NAT timeout, wifi loss, sleep) left
   ghost users forever. Fix: client Ping every 15s with RTT measurement,
   ConnSession::last_seen bumped on every inbound TCP/UDP frame, steady_timer
   reaper sweeps every 15s and drops sessions older than 45s (configurable
   via server::Config).

4. UDP KEEPALIVE: client sends plaintext kFrameKeepalive every 5s; server
   bumps last_seen + echoes back. Keeps NAT bindings alive and lets media
   activity defer the reaper independently of TCP.

5. Graceful client disconnect: vc_disconnect() sends Disconnect{code=0} via
   a flag-based io-thread exit (no double-close race); server handles
   client-sent Disconnect with immediate close() + LEFT broadcast.

3 new tests: disconnect_left, plc_cap, reaper_timeout. 21/21 ctest green.

Docs: protocol.md §6/§7, voice.md §6, architecture.md §5, PROGRESS.md.

docs/voice.md

@@ -137,7 +137,12 @@ Each receiver keeps an **adaptive jitter buffer per ssrc**.
 ## 6. UDP keepalive & NAT
 
 - A `KEEPALIVE` (type 2) frame flows both directions on the media channel every ~5 s to
-  hold NAT bindings and measure media-path RTT/loss independent of TCP.
+  hold NAT bindings and measure media-path RTT/loss independent of TCP. The frame is
+  plaintext (14-byte header, no payload, no AEAD) — the server identifies the sender by
+  its already-verified UDP endpoint (established during the `UdpBinding` handshake). On
+  receipt the server bumps the sender's `last_seen` (so media activity defers the TCP
+  reaper independently of control-channel traffic) and echoes the frame back so the
+  client can measure media-path RTT.
 - If the media path dies but TCP is alive, the client surfaces a "voice disconnected"
   state and attempts UDP re-binding (re-derive media keys + fresh `UdpBinding`) without dropping
   the control session.