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/cli.rs

@@ -0,0 +1,103 @@
+use clap::{Parser, ValueEnum};
+
+#[derive(Parser, Debug)]
+#[command(
+    name = "pixelpass",
+    version,
+    about = "P2P screen sharing over iroh + ffmpeg",
+    long_about = "Run with no arguments to host (prints a ticket to share). \
+                  Pass a ticket to view."
+)]
+pub struct Cli {
+    /// iroh ticket. If present, runs as viewer. If absent, runs as host.
+    pub ticket: Option<String>,
+
+    // ── host options ──────────────────────────────────────────────────
+    /// Pick a single window instead of the whole screen.
+    #[arg(long)]
+    pub window: bool,
+
+    /// Capture only this app's audio (per-app PipeWire routing).
+    #[arg(long, value_name = "NAME")]
+    pub app: Option<String>,
+
+    /// Mix in the default microphone source.
+    #[arg(long)]
+    pub mic: bool,
+
+    /// Override display server autodetection.
+    #[arg(long, value_enum)]
+    pub display_server: Option<DisplayServerArg>,
+
+    /// Encode bitrate in kbps.
+    #[arg(long, default_value_t = 6000)]
+    pub bitrate: u32,
+
+    /// Capture framerate.
+    #[arg(long, default_value_t = 30)]
+    pub framerate: u32,
+
+    /// Disable VAAPI HW encode; force software x264.
+    #[arg(long)]
+    pub no_hwencode: bool,
+
+    /// Use low-latency SRT transport instead of HTTP MPEG-TS (Phase 2/3).
+    #[arg(long)]
+    pub low_latency: bool,
+
+    // ── viewer options ────────────────────────────────────────────────
+    /// Local TCP port for the viewer to expose (default: random).
+    #[arg(long, default_value_t = 0)]
+    pub port: u16,
+
+    // ── global ────────────────────────────────────────────────────────
+    /// Trace-level logging.
+    #[arg(long, short)]
+    pub verbose: bool,
+
+    /// Clean up orphaned PipeWire state from a crashed host run, then exit.
+    #[arg(long)]
+    pub repair: bool,
+}
+
+#[derive(ValueEnum, Clone, Copy, Debug)]
+pub enum DisplayServerArg {
+    Wayland,
+    X11,
+}
+
+#[derive(Debug, Clone)]
+pub struct HostOpts {
+    pub window: bool,
+    pub app: Option<String>,
+    pub mic: bool,
+    pub display_server: Option<DisplayServerArg>,
+    pub bitrate: u32,
+    pub framerate: u32,
+    pub no_hwencode: bool,
+    pub low_latency: bool,
+}
+
+#[derive(Debug, Clone)]
+pub struct ViewerOpts {
+    pub port: u16,
+}
+
+impl Cli {
+    pub fn into_host_opts(self) -> HostOpts {
+        HostOpts {
+            window: self.window,
+            app: self.app,
+            mic: self.mic,
+            display_server: self.display_server,
+            bitrate: self.bitrate,
+            framerate: self.framerate,
+            no_hwencode: self.no_hwencode,
+            low_latency: self.low_latency,
+        }
+    }
+
+    pub fn into_viewer_opts(self) -> ViewerOpts {
+        ViewerOpts { port: self.port }
+    }
+}