Build only one window layout, so the eyes stop jumping

Main.qml declared both layouts at once: the small per-object windows and the
full-screen overlay. Both of them write the controller's catCenter, which is
where the eyes look for the cursor. Two bindings on one property overwrite each
other, so catCenter alternated every frame between the cat window's real centre
and the hidden overlay's, whose height is zero. look.y flipped between -0.495
and 1.0 sixty times a second and the pupils jumped with it.

Each layout now sits behind its own Loader and only the one the platform needs
is built. The small windows moved into Desktop.qml so a single component can
hold all three of them.

Measured in the running app while walking, the eye's vertical travel went from
2.6 px to 0.40 px, and look.y is now a single steady value. It also stops a
second, invisible cat from animating in the background.

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

CMakeLists.txt

@@ -22,7 +22,7 @@ qt_add_executable(tncat
 target_include_directories(tncat PRIVATE src)
 
 # QML files live in qml/ but register at the module root (TNCat.Main etc.)
-set(TNCAT_QML Main Cat Chip Cup CatWindow ChipWindow CupWindow OverlayWindow
+set(TNCAT_QML Main Desktop Cat Chip Cup CatWindow ChipWindow CupWindow OverlayWindow
                SettingsWindow AppButton AppSlider AppToggle SettingsGroup CatDefs)
 foreach(qmlfile IN LISTS TNCAT_QML)
     set_source_files_properties(qml/${qmlfile}.qml PROPERTIES QT_RESOURCE_ALIAS ${qmlfile}.qml)

qml/Desktop.qml

@@ -0,0 +1,31 @@
+pragma ComponentBehavior: Bound
+import QtQuick
+import TNCat
+
+// The layout for platforms that let a client place its own window: one small
+// window per object, moved around the screen by the app.
+QtObject {
+    id: desktop
+
+    required property CatController cat
+    required property AppSettings settings
+
+    signal settingsRequested()
+
+    readonly property CatWindow catWindow: CatWindow {
+        cat: desktop.cat
+        settings: desktop.settings
+        visible: true
+        onSettingsRequested: desktop.settingsRequested()
+    }
+
+    readonly property ChipWindow chipWindow: ChipWindow {
+        cat: desktop.cat
+        settings: desktop.settings
+    }
+
+    readonly property CupWindow cupWindow: CupWindow {
+        cat: desktop.cat
+        settings: desktop.settings
+    }
+}

qml/Main.qml

@@ -2,7 +2,9 @@ pragma ComponentBehavior: Bound
 import QtQuick
 import TNCat
 
-// Root is not a window: which windows exist depends on the platform.
+// Root is not a window. Exactly one of the two layouts is built, because both
+// of them drive the controller's catCenter; having both alive made the two
+// bindings overwrite each other every frame and the eyes jumped with them.
 QtObject {
     id: app
 
@@ -15,31 +17,24 @@ QtObject {
         settingsWindow.requestActivate()
     }
 
-    readonly property CatWindow catWindow: CatWindow {
+    readonly property Loader desktopLoader: Loader {
+        active: !Platform.overlayMode
+        sourceComponent: Desktop {
             cat: app.cat
             settings: app.settings
-        visible: !Platform.overlayMode
             onSettingsRequested: app.openSettings()
         }
-
-    readonly property ChipWindow chipWindow: ChipWindow {
-        cat: app.cat
-        settings: app.settings
-        visible: !Platform.overlayMode && app.cat.chipVisible
     }
 
-    readonly property CupWindow cupWindow: CupWindow {
+    readonly property Loader overlayLoader: Loader {
+        active: Platform.overlayMode
+        sourceComponent: OverlayWindow {
             cat: app.cat
             settings: app.settings
-        visible: !Platform.overlayMode && app.cat.cupVisible
+            overlayActive: true
-    }
-
-    readonly property OverlayWindow overlayWindow: OverlayWindow {
-        cat: app.cat
-        settings: app.settings
-        overlayActive: Platform.overlayMode
             onSettingsRequested: app.openSettings()
         }
+    }
 
     readonly property SettingsWindow settingsWindow: SettingsWindow {
         cat: app.cat