feat(touch): cross-client touch-input modes on Linux + Windows

Bring the SDL presenter (Linux/Deck + Windows) to parity with the Android and
Apple clients: a persisted TouchMode selects how a touchscreen drives the host —

  * Trackpad (default): relative cursor with pointer ballistics + the shared
    gesture vocabulary (tap = left click, two-finger tap = right click,
    two-finger drag = scroll, tap-then-drag = held left drag, three-finger tap =
    cycle the stats overlay).
  * Direct pointer: the cursor jumps to and follows the finger (absolute).
  * Touch passthrough: every finger is a real host touchscreen contact.

Previously the presenter had no finger handling, so SDL synthesized mouse events
from touch and — under the stream's relative-mouse lock — walked the host cursor
into the corner (the reported Deck bug). SDL touch->mouse synthesis is now off;
DIRECT touchscreens route through a new incremental gesture engine (a port of
Android TouchInput.kt / Apple TouchMouse.swift), while INDIRECT trackpads keep
driving the mouse. Fingers map through the aspect-fit letterbox onto the content
rect.

TouchMode lives in the shared trust::Settings (default trackpad, so passthrough
is opt-in like the other clients); the GTK and WinUI settings screens both gained
a "Touch input" picker. Gesture engine, letterbox mapping, and settings
back-compat are unit-tested (28 tests green); clippy -D warnings clean; full
Linux client + session build verified on-host.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

clients/windows/src/app/settings.rs

@@ -55,6 +55,13 @@ const STATS_TIERS: &[(StatsVerbosity, &str)] = &[
     (StatsVerbosity::Normal, "Normal"),
     (StatsVerbosity::Detailed, "Detailed"),
 ];
+/// Touch-input presets: `(stored value, display label)` — how a touchscreen's fingers drive
+/// the host. The cross-client set (Android/Apple); only meaningful on a touchscreen device.
+const TOUCH_MODES: &[(&str, &str)] = &[
+    ("trackpad", "Trackpad"),
+    ("pointer", "Direct pointer"),
+    ("touch", "Touch passthrough"),
+];
 /// Host compositor presets: `(stored value, display label)`. Advisory — the host falls back to
 /// auto-detect when the choice is unavailable. Only meaningful against a Linux host.
 const COMPOSITORS: &[(&str, &str)] = &[
@@ -324,6 +331,14 @@ pub(crate) fn settings_page(
         "The virtual pad the host creates. \u{201C}Automatic\u{201D} matches your physical \
          controller.",
     );
+    let (touch_names, touch_i) = presets(TOUCH_MODES, |v| *v == s.touch_mode);
+    let touch_combo = setting_combo(ctx, "Touch input", touch_names, touch_i, |s, i| {
+        s.touch_mode = TOUCH_MODES[i].0.to_string();
+    })
+    .tooltip(
+        "How a touchscreen drives the host: Trackpad nudges a cursor (tap to click), Direct \
+         pointer jumps to your finger, Touch passthrough sends real touches.",
+    );
     let shortcuts_toggle = setting_toggle(
         ctx,
         "Capture system shortcuts (Alt+Tab, Win, \u{2026})",
@@ -405,6 +420,7 @@ pub(crate) fn settings_page(
             settings_card(vec![
                 forward_combo.into(),
                 pad_combo.into(),
+                touch_combo.into(),
                 shortcuts_toggle.into(),
             ]),
         ),