Basic shell working.

GUIInputHandler.h

@@ -0,0 +1,31 @@
+/*
+ * GUIInputHandler - ImGui/SDL2-based input mapping for GUI mode
+ */
+#ifndef KTE_GUI_INPUT_HANDLER_H
+#define KTE_GUI_INPUT_HANDLER_H
+
+#include <queue>
+#include <mutex>
+
+#include "InputHandler.h"
+
+union SDL_Event; // fwd decl to avoid including SDL here (SDL defines SDL_Event as a union)
+
+class GUIInputHandler : public InputHandler {
+public:
+    GUIInputHandler() = default;
+    ~GUIInputHandler() override = default;
+
+    // Translate an SDL event to editor command and enqueue if applicable.
+    // Returns true if it produced a mapped command or consumed input.
+    bool ProcessSDLEvent(const SDL_Event &e);
+
+    bool Poll(MappedInput &out) override;
+
+private:
+    std::mutex mu_;
+    std::queue<MappedInput> q_;
+    bool k_prefix_ = false;
+};
+
+#endif // KTE_GUI_INPUT_HANDLER_H