feat: add WebRTC streaming via str0m + portal session persistence

- Add src/webrtc.rs: HTTP signaling server + str0m Sans-IO WebRTC transport
  with H.264 Annex-B → RTP packetization and key-frame request handling
- avhw: introduce FrameOutput enum (Muxer | Channel) so SwEncState can
  output to either MP4 muxer or crossbeam channel for WebRTC
- cap_portal: support portal session restore tokens (PersistMode::ExplicitlyRevoked)
  to skip re-authorization dialog; add --no-persist flag to force fresh dialog
- args: make --output optional when --port is used for WebRTC mode
- state_portal: integrate WebRTC pipeline (encoder channel → RTP forwarding)
  with shorter GOP for WebRTC (fps/2, min 10)
- main: redirect tracing to stderr; validate --output or --port required
- Add dependencies: str0m 0.20, serde_json 1, dirs 6

src/args.rs

@@ -3,9 +3,9 @@ use clap::Parser;
 #[derive(Parser, Debug, Clone)]
 #[command(name = "wl-webrtc", about = "Wayland screen capture and encoding tool")]
 pub struct Args {
-    /// Output file path (e.g., output.mp4, output.mkv)
+    /// Output file path (e.g., output.mp4, output.mkv). Optional when using --port for WebRTC mode
     #[arg(short, long)]
-    pub output: String,
+    pub output: Option<String>,
 
     /// Wayland output name to capture
     #[arg(long)]
@@ -43,7 +43,11 @@ pub struct Args {
     #[arg(long)]
     pub backend: Option<String>,
 
-    /// Port for WebTransport server (Phase 2, unused in MVP)
+    /// Port for WebRTC HTTP signaling server; 0 keeps MP4 file output mode
     #[arg(long, default_value_t = 0)]
     pub port: u16,
+
+    /// Force re-authorization dialog (ignore saved portal restore token)
+    #[arg(long)]
+    pub no_persist: bool,
 }