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.

include/screencast/app/cli.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cstdint>
 #include <optional>
 #include <string_view>
 #include <variant>
@@ -7,17 +8,22 @@
 namespace sc {
 
 struct SendCommand {
-    std::string_view target = "monitor"; // monitor, window, region
-    std::string_view peer_address;       // optional
+    std::string_view target = "monitor"; // monitor, window
+    std::string_view peer_address;       // optional; empty means auto-discover
     int bitrate_kbps = 4000;
 };
 
 struct ReceiveCommand {
-    std::string_view peer_address; // optional
+    std::string_view peer_address; // optional, informational
     int local_rtp_port = 5004;
+    int signaling_port = 5005;
 };
 
-using Command = std::variant<SendCommand, ReceiveCommand>;
+struct DiscoverCommand {
+    int timeout_seconds = 3;
+};
+
+using Command = std::variant<SendCommand, ReceiveCommand, DiscoverCommand>;
 
 // Parse command line arguments. Prints usage and returns std::nullopt on error.
 // `argv` is `char const* const*` so both `main`'s `char**` and const arrays

include/screencast/app/pipeline.h

@@ -26,6 +26,8 @@ struct ReceiverPipelineConfig {
     Endpoint local_rtp_endpoint;
     std::optional<Endpoint> peer_signaling;
     RendererConfig renderer;
+    // TCP port the signaling server listens on (offers arrive here).
+    std::uint16_t signaling_port = 5005;
 };
 
 // Sender pipeline: capture → encode → packetize → RTP/UDP.