feat(engine): UI scale setting — 90/100/115/130% (Phase K)

Settings -> Accessibility gains a UI Scale row cycling four steps,
applied live through bevy::ui::UiScale: every menu, modal, and HUD
element scales while the table itself stays window-fit via
compute_layout. New Settings::ui_scale (serde default 1.0, sanitized
clamp to [0.9, 1.3]).

Safe-area anchors and modal scrim padding pre-divide physical insets
by the UI scale so post-multiplication lands exactly on the system
bars — without this, 90% would sink the bottom action bar into the
Android gesture zone. The anchor systems also re-run on UiScale
changes, not just inset changes.

Deferred from Phase K (noted in the doc): seeding the setting from the
Android system font scale on first run (needs JNI), and the 44px
touch-target audit (on-device).

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

solitaire_engine/src/settings_plugin/tests.rs

@@ -736,3 +736,60 @@ fn scroll_clamps_offset_to_zero_at_top() {
         "scrolling past top must clamp to 0, got {offset}"
     );
 }
+
+// ---------------------------------------------------------------------------
+// Phase K: UI scale
+// ---------------------------------------------------------------------------
+
+#[test]
+fn ui_scale_steps_cycle_and_wrap() {
+    assert_eq!(next_ui_scale(0.9), 1.0);
+    assert_eq!(next_ui_scale(1.0), 1.15);
+    assert_eq!(next_ui_scale(1.15), 1.3);
+    assert_eq!(next_ui_scale(1.3), 0.9, "the cycle must wrap");
+    // A hand-edited in-between value advances to the next larger step.
+    assert_eq!(next_ui_scale(1.05), 1.15);
+}
+
+#[test]
+fn ui_scale_label_formats_as_percent() {
+    assert_eq!(ui_scale_label(0.9), "90%");
+    assert_eq!(ui_scale_label(1.0), "100%");
+    assert_eq!(ui_scale_label(1.15), "115%");
+    assert_eq!(ui_scale_label(1.3), "130%");
+}
+
+#[test]
+fn ui_scale_setting_syncs_the_bevy_resource() {
+    let mut app = headless_app();
+    app.insert_resource(UiScale(1.0));
+    app.world_mut()
+        .resource_mut::<SettingsResource>()
+        .0
+        .ui_scale = 1.3;
+    app.update();
+
+    assert!(
+        (app.world().resource::<UiScale>().0 - 1.3).abs() < f32::EPSILON,
+        "Settings::ui_scale must drive bevy::ui::UiScale"
+    );
+}
+
+#[test]
+fn ui_scale_out_of_range_sanitizes_on_load() {
+    use solitaire_data::settings::{UI_SCALE_MAX, UI_SCALE_MIN};
+
+    let wild = Settings {
+        ui_scale: 5.0,
+        ..Settings::default()
+    }
+    .sanitized();
+    assert_eq!(wild.ui_scale, UI_SCALE_MAX);
+
+    let tiny = Settings {
+        ui_scale: 0.1,
+        ..Settings::default()
+    }
+    .sanitized();
+    assert_eq!(tiny.ui_scale, UI_SCALE_MIN);
+}