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/KeyGeometry.cpp

@@ -0,0 +1,46 @@
+#include "KeyGeometry.h"
+
+#include <algorithm>
+
+namespace {
+
+qreal rowUnits(const Row &row)
+{
+    qreal units = 0;
+    for (const KeyDef &key : row)
+        units += key.width;
+    return units;
+}
+
+} // namespace
+
+QList<QRectF> computeKeyRects(const Layer &layer, QSizeF size, KeyboardMetrics metrics)
+{
+    QList<QRectF> rects;
+    if (layer.rows.isEmpty())
+        return rects;
+
+    qreal maxUnits = 0;
+    for (const Row &row : layer.rows)
+        maxUnits = std::max(maxUnits, rowUnits(row));
+
+    const qreal innerWidth = size.width() - 2 * metrics.padding;
+    const qreal innerHeight = size.height() - 2 * metrics.padding;
+    const int rowCount = int(layer.rows.size());
+
+    // Each cell includes one trailing gap; the last gap of a row is dropped.
+    const qreal unitWidth = (innerWidth + metrics.gap) / maxUnits;
+    const qreal rowHeight = (innerHeight - (rowCount - 1) * metrics.gap) / rowCount;
+
+    qreal y = metrics.padding;
+    for (const Row &row : layer.rows) {
+        qreal x = metrics.padding + (maxUnits - rowUnits(row)) * unitWidth / 2;
+        for (const KeyDef &key : row) {
+            const qreal cellWidth = key.width * unitWidth;
+            rects.append(QRectF(x, y, std::max<qreal>(0, cellWidth - metrics.gap), rowHeight));
+            x += cellWidth;
+        }
+        y += rowHeight + metrics.gap;
+    }
+    return rects;
+}