feat(network): implement Phase 6 LAN discovery and session signaling

Add mDNS/DNS-SD discovery and JSON session negotiation so two peers on
a LAN connect without hard-coded addresses.

- Discovery (Avahi threaded-poll client): the receiver announces
  _screencast._tcp with its signaling port; senders browse and resolve
  peers. Strict lock ordering (poll lock before state mutex) keeps the
  callbacks deadlock-free; name collisions rename via
  avahi_alternative_service_name.
- Signaling: one JSON object per newline-terminated TCP line. The
  receiver hosts a server (port 5005) and answers session offers with
  its RTP port; senders connect, offer, and stream to the negotiated
  endpoint. WebSocket was deferred: no WS library is installed, the
  skill permits plain TCP, and the wire format is transport-agnostic.
- Dual-stack transports: this machine resolves its own services over
  IPv6, so getaddrinfo now runs AF_UNSPEC and listeners bind IPv6 with
  IPV6_V6ONLY=0 (IPv4 fallback), covering UDP and TCP alike. The
  signaling client shutdown now uses shutdown() so a reader blocked in
  recv() cannot hang the join (a plain close() does not wake it).
- CLI: --discover lists receivers (deduped to one entry per host);
  --send auto-disovers when exactly one receiver is found; --peer
  targets a receiver directly; --signaling-port overrides the default.

Validation: meson test 5/5 (new signaling round-trip test), valgrind
clean. End-to-end over loopback: --discover finds the announced
receiver, a probe negotiated a session and streamed 60 frames over
both IPv4 and IPv6, and the receiver reported stream started. mDNS
resolution was verified against avahi-browse as an independent
reference.

tests/meson.build

@@ -21,3 +21,9 @@ test_loopback = executable('test_loopback',
     dependencies : [sc_codec_dep, sc_network_dep])
 
 test('udp loopback', test_loopback)
+
+test_signaling = executable('test_signaling',
+    'network/test_signaling.cpp',
+    dependencies : sc_network_dep)
+
+test('session signaling', test_signaling)