Layouts, iPad-style rendering, taps, shift and layers

- JSON layouts (en-US, sv-SE inheriting number/symbol layers via base)
- LayoutModel + KeyGeometry: proportional rows, nearest-key hit testing
- KeyboardSurface: multi-pointer touch/mouse routing to the controller
- KeyboardController + ShiftState: iOS shift/caps-lock, layers, globe, dismiss
- QML view with light/dark iPad palette and drag handle
- DevHooks: SLOPOSK_AUTOTYPE, SLOPOSK_SCREENSHOT for headless checks
- Unit tests for shift, loader, geometry and controller

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/layout/Layout.h

@@ -0,0 +1,59 @@
+#pragma once
+
+#include <QList>
+#include <QString>
+#include <QStringList>
+
+enum class KeyType {
+    Character,
+    Space,
+    Backspace,
+    Enter,
+    Shift,
+    LayerSwitch,
+    Globe,
+    Dismiss,
+    Spacer, // empty room, e.g. the half-key indent of the home row
+};
+
+struct KeyDef {
+    KeyType type = KeyType::Character;
+    QString label;          // shown and typed (for character keys)
+    QString shifted;        // explicit shifted form; empty means label.toUpper()
+    QString flick;          // secondary symbol, typed by flicking down
+    QStringList alternates; // long-press choices
+    QString target;         // layer to switch to (LayerSwitch only)
+    qreal width = 1.0;      // in key units
+
+    // Text this key shows/types; only character keys change case.
+    QString text(bool uppercase) const
+    {
+        if (!uppercase || type != KeyType::Character)
+            return label;
+        return shifted.isEmpty() ? label.toUpper() : shifted;
+    }
+};
+
+using Row = QList<KeyDef>;
+
+struct Layer {
+    QString name;
+    QList<Row> rows;
+};
+
+struct Layout {
+    QString id;      // e.g. "en-US"
+    QString name;    // human readable
+    QString locale;  // e.g. "en_US"
+    QString base;    // id of a layout to take missing layers from
+    QList<Layer> layers; // the first one is shown by default
+
+    const Layer *layer(const QString &layerName) const
+    {
+        for (const Layer &l : layers) {
+            if (l.name == layerName)
+                return &l;
+        }
+        return nullptr;
+    }
+};