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/engine/InputEngine.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <QString>
+
+// Turns what the keyboard produces into text operations.
+// DirectEngine passes them straight through; prediction and swipe engines
+// will sit here too (preedit, candidates, auto-correction).
+class InputEngine
+{
+public:
+    virtual ~InputEngine() = default;
+
+    virtual void commitText(const QString &text) = 0;
+    virtual void sendKeysym(quint32 keysym) = 0;
+};