Phase 1 foundation: CLI, iroh tunnel, lazy capture wiring

Scaffolding for PixelPass per ~/Documents/p2p-screenshare-plan.md §7
"Phase 1 — MVP". The full QUIC tunnel handshake works end-to-end
(verified locally: host generates ticket, viewer dials through iroh's
relay, open_bi succeeds, lazy capture is wired correctly).

What's implemented:

- Cargo project with deps locked: iroh 1.0.0-rc.0, iroh-tickets,
  tokio, clap, ashpd, pipewire-rs, x11rb, ashpd, anyhow, thiserror,
  tracing, nix, directories, uuid.
- src/cli.rs: complete clap surface per plan §6 (--window, --app,
  --mic, --display-server, --bitrate, --framerate, --no-hwencode,
  --low-latency, --port, --verbose, --repair).
- Mode dispatch in main.rs: EndpointTicket::from_str is the
  authoritative check; no regex / heuristics.
- common/display.rs: WAYLAND_DISPLAY → DISPLAY → XDG_SESSION_TYPE
  precedence with --display-server override.
- common/deps.rs: per-distro install hints (pacman/apt/dnf/zypper)
  parsing /etc/os-release.
- common/alpn.rs: ALPN = b"pixelpass/0".
- common/tunnel.rs: generic bidirectional bridge between an iroh
  bi-stream and any AsyncRead+AsyncWrite (typically a TCP socket).
- common/signal.rs: ctrl-c -> CancellationToken; second ctrl-c hard
  exit.
- host/mod.rs: build Endpoint, generate ticket, print banner, await
  first peer (lazy — no ffmpeg until peer connects), accept_bi,
  spawn capture, bridge to localhost ffmpeg HTTP listener.
- host/capture.rs: stub returning Phase-2 error; the place X11
  x11grab and Wayland ashpd+gst pipelines will land.
- viewer/mod.rs: Endpoint, connect with ALPN, open_bi, TcpListener
  on 127.0.0.1, print copy-ready mpv/vlc commands, bridge.
- repair.rs: stub for --repair PipeWire scan.

iroh 1.0-rc renamed Node* -> Endpoint* and moved EndpointTicket into
a sibling crate (iroh-tickets); no design impact. Plan still locked.

src/main.rs

@@ -0,0 +1,40 @@
+mod cli;
+mod common;
+mod host;
+mod repair;
+mod viewer;
+
+use anyhow::Result;
+use clap::Parser;
+use cli::Cli;
+use iroh_tickets::endpoint::EndpointTicket;
+use tracing_subscriber::EnvFilter;
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    let cli = Cli::parse();
+    init_tracing(cli.verbose);
+
+    if cli.repair {
+        return repair::run().await;
+    }
+
+    match cli.ticket.as_deref() {
+        Some(s) => {
+            let ticket: EndpointTicket = s.parse().map_err(|e| {
+                anyhow::anyhow!(
+                    "argument doesn't look like a pixelpass ticket ({e}).\n\
+                     Run with no arguments to host, or pass a ticket to view."
+                )
+            })?;
+            viewer::run(ticket, cli.into_viewer_opts()).await
+        }
+        None => host::run(cli.into_host_opts()).await,
+    }
+}
+
+fn init_tracing(verbose: bool) {
+    let default = if verbose { "pixelpass=trace,iroh=info" } else { "pixelpass=info,iroh=warn" };
+    let filter = EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new(default));
+    tracing_subscriber::fmt().with_env_filter(filter).with_target(false).init();
+}