feat(client-linux): native GTK4 client — stage 1, first light at 1080p60

New crate crates/punktfunk-client-linux (binary punktfunk-client), the
native Linux client on the Option A architecture (2026-06-12 research):

- GTK4/libadwaita shell linking punktfunk-core directly (no C ABI):
  mDNS host list, TOFU fingerprint prompt, SPAKE2 PIN pairing dialog,
  preferences (mode/bitrate/gamepad/shortcut capture), stats overlay,
  --connect host[:port] for scripting.
- Video: FFmpeg software HEVC decode (LOW_DELAY, slice threads) ->
  RGBA -> GdkMemoryTexture inside GtkGraphicsOffload (the dmabuf
  subsurface path lights up when VAAPI lands; black-background keeps
  fullscreen scanout-eligible).
- Audio: Opus -> PipeWire playback stream, the host virtual-mic's
  adaptive jitter ring inverted.
- Input: keyboard as the exact inverse of the host VK table (evdev
  keycodes, layout-independent; unit-tested), absolute mouse through
  the Contain-fit transform, WHEEL_DELTA(120) scroll, compositor
  shortcut inhibition while streaming, Ctrl+Alt+Shift+Q release chord,
  F11 fullscreen. SDL3 gamepad capture (single pad-0 model) + rumble
  and DualSense lightbar feedback on the same thread.
- Session pump owns video+audio pulls; the gamepad thread owns
  rumble+hidout — possible because NativeClient's plane receivers are
  now mutexed, making it Sync (Arc-shared, compiler-verified per-plane
  contract instead of the ABI's manual assertion).
- Linux-gated deps + a stub main keep cargo build --workspace green on
  macOS.

Validated live against serve --native on this box: 1920x1080@60,
locked 60 fps, capture->decoded p50 ~6.4 ms (software decode, debug
build). Teardown keys off AdwNavigationPage::hidden — NavigationView
push fires a transient unmap/map cycle that must not end the session.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-client-linux/src/main.rs

@@ -0,0 +1,42 @@
+//! `punktfunk-client` — the native Linux punktfunk/1 client (design: Option A, 2026-06-12).
+//!
+//! GTK4/libadwaita shell · `NativeClient` linked as a crate (no C ABI) · FFmpeg decode →
+//! `GtkGraphicsOffload` present · PipeWire audio · SDL3 gamepads. The trust surface
+//! mirrors the Apple client: persistent identity, TOFU prompt with the host fingerprint,
+//! SPAKE2 PIN pairing.
+
+#[cfg(target_os = "linux")]
+mod app;
+#[cfg(target_os = "linux")]
+mod audio;
+#[cfg(target_os = "linux")]
+mod discovery;
+#[cfg(target_os = "linux")]
+mod gamepad;
+#[cfg(target_os = "linux")]
+mod keymap;
+#[cfg(target_os = "linux")]
+mod session;
+#[cfg(target_os = "linux")]
+mod trust;
+#[cfg(target_os = "linux")]
+mod ui_hosts;
+#[cfg(target_os = "linux")]
+mod ui_settings;
+#[cfg(target_os = "linux")]
+mod ui_stream;
+#[cfg(target_os = "linux")]
+mod video;
+
+#[cfg(target_os = "linux")]
+fn main() -> gtk::glib::ExitCode {
+    app::run()
+}
+
+/// GTK4/PipeWire/SDL3 are Linux turf; this stub keeps `cargo build --workspace` green on
+/// macOS (the Mac client lives in clients/apple).
+#[cfg(not(target_os = "linux"))]
+fn main() {
+    eprintln!("punktfunk-client is Linux-only — the macOS client lives in clients/apple");
+    std::process::exit(2);
+}