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/ui_settings.rs

@@ -0,0 +1,93 @@
+//! Preferences dialog: stream mode, bitrate, gamepad type, capture behavior. Written
+//! back to disk when the dialog closes.
+
+use crate::trust::Settings;
+use adw::prelude::*;
+use std::cell::RefCell;
+use std::rc::Rc;
+
+const RESOLUTIONS: &[(u32, u32)] = &[(1280, 720), (1920, 1080), (2560, 1440), (3840, 2160)];
+const REFRESH: &[u32] = &[30, 60, 90, 120, 144, 165, 240];
+const GAMEPADS: &[&str] = &["auto", "xbox360", "dualsense"];
+
+pub fn show(parent: &impl IsA<gtk::Widget>, settings: Rc<RefCell<Settings>>) {
+    let page = adw::PreferencesPage::new();
+
+    let stream = adw::PreferencesGroup::builder().title("Stream").build();
+    let res_names: Vec<String> = RESOLUTIONS
+        .iter()
+        .map(|(w, h)| format!("{w} × {h}"))
+        .collect();
+    let res_row = adw::ComboRow::builder()
+        .title("Resolution")
+        .subtitle("The host creates a virtual output at exactly this size")
+        .model(&gtk::StringList::new(
+            &res_names.iter().map(String::as_str).collect::<Vec<_>>(),
+        ))
+        .build();
+    let hz_row = adw::ComboRow::builder()
+        .title("Refresh rate")
+        .model(&gtk::StringList::new(
+            &REFRESH
+                .iter()
+                .map(|r| format!("{r} Hz"))
+                .collect::<Vec<_>>()
+                .iter()
+                .map(String::as_str)
+                .collect::<Vec<_>>(),
+        ))
+        .build();
+    let bitrate_row = adw::SpinRow::with_range(0.0, 500.0, 5.0);
+    bitrate_row.set_title("Bitrate");
+    bitrate_row.set_subtitle("Mbit/s · 0 = host default");
+    stream.add(&res_row);
+    stream.add(&hz_row);
+    stream.add(&bitrate_row);
+
+    let input = adw::PreferencesGroup::builder().title("Input").build();
+    let pad_row = adw::ComboRow::builder()
+        .title("Gamepad type")
+        .subtitle("The virtual pad the host creates (DualSense needs a Linux host)")
+        .model(&gtk::StringList::new(&["Auto", "Xbox 360", "DualSense"]))
+        .build();
+    let inhibit_row = adw::SwitchRow::builder()
+        .title("Capture system shortcuts")
+        .subtitle("Forward Alt+Tab, Super, … to the host while streaming")
+        .build();
+    input.add(&pad_row);
+    input.add(&inhibit_row);
+
+    page.add(&stream);
+    page.add(&input);
+
+    // Seed from the current settings.
+    {
+        let s = settings.borrow();
+        let res_i = RESOLUTIONS
+            .iter()
+            .position(|&(w, h)| w == s.width && h == s.height)
+            .unwrap_or(1);
+        res_row.set_selected(res_i as u32);
+        let hz_i = REFRESH.iter().position(|&r| r == s.refresh_hz).unwrap_or(1);
+        hz_row.set_selected(hz_i as u32);
+        bitrate_row.set_value(f64::from(s.bitrate_kbps) / 1000.0);
+        let pad_i = GAMEPADS.iter().position(|&g| g == s.gamepad).unwrap_or(0);
+        pad_row.set_selected(pad_i as u32);
+        inhibit_row.set_active(s.inhibit_shortcuts);
+    }
+
+    let dialog = adw::PreferencesDialog::new();
+    dialog.set_title("Preferences");
+    dialog.add(&page);
+    dialog.connect_closed(move |_| {
+        let mut s = settings.borrow_mut();
+        let (w, h) = RESOLUTIONS[(res_row.selected() as usize).min(RESOLUTIONS.len() - 1)];
+        (s.width, s.height) = (w, h);
+        s.refresh_hz = REFRESH[(hz_row.selected() as usize).min(REFRESH.len() - 1)];
+        s.bitrate_kbps = (bitrate_row.value() * 1000.0) as u32;
+        s.gamepad = GAMEPADS[(pad_row.selected() as usize).min(GAMEPADS.len() - 1)].to_string();
+        s.inhibit_shortcuts = inhibit_row.is_active();
+        s.save();
+    });
+    dialog.present(Some(parent));
+}