fix(core): resolve window split freezing with proper focus API

The editor was freezing when splits were triggered due to infinite loops
in window focusing logic when trying to find windows after layout rebuilds.

Key changes:

1. **Added set_active_window method**: Proper API in EditorCore to set active
   window by validating it exists in current window tree

2. **Fixed infinite loops**: Replaced hacky next_window() cycling with direct
   set_active_window() calls in GTK focus handlers

3. **Exposed to Lua**: Added set_active_window to Lua API for completeness

4. **Simplified focus handling**: Removed window cycling from key handlers,
   only using focus controller with proper validation

This resolves the freeze and allows proper window splits and focus management.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

include/lumacs/editor_core.hpp

@@ -131,6 +131,9 @@ public:
 
     /// Get the active window
     std::shared_ptr<Window> active_window() const { return active_window_; }
+    
+    /// Set the active window (if it exists in the window tree)
+    bool set_active_window(std::shared_ptr<Window> window);
 
     /// Get the root of the layout tree (for rendering)
     std::shared_ptr<LayoutNode> root_layout() const { return root_node_; }

src/editor_core.cpp

@@ -373,6 +373,25 @@ void EditorCore::next_window() {
     }
 }
 
+bool EditorCore::set_active_window(std::shared_ptr<Window> window) {
+    if (!window) return false;
+    
+    // Verify that the window exists in the current window tree
+    std::vector<std::shared_ptr<Window>> windows;
+    collect_windows(root_node_.get(), windows);
+    
+    auto it = std::find(windows.begin(), windows.end(), window);
+    if (it != windows.end()) {
+        if (active_window_ != window) {
+            active_window_ = window;
+            emit_event(EditorEvent::WindowFocused);
+        }
+        return true;
+    }
+    
+    return false; // Window not found in tree
+}
+
 // === Cursor Proxies ===
 
 Position EditorCore::cursor() const noexcept {

src/gtk_editor.cpp

@@ -397,15 +397,9 @@ protected:
             std::weak_ptr<Window> weak_window_focus = node->window;
             focus_controller->signal_enter().connect([this, weak_window_focus]() {
                 if (auto window = weak_window_focus.lock()) {
-                    if (core_ && core_->active_window() != window) {
-                        // Only change focus if this window is not already active
-                        // Try to find the target window by calling next_window repeatedly
-                        for (int i = 0; i < 10; ++i) { // Prevent infinite loop
-                            core_->next_window();
-                            if (core_->active_window() == window) {
-                                break;
-                            }
-                        }
+                    if (core_) {
+                        // Use the proper set_active_window method
+                        core_->set_active_window(window);
                     }
                 }
             });

src/lua_api.cpp

@@ -415,6 +415,7 @@ void LuaApi::register_types() {
         "split_vertically", &EditorCore::split_vertically,
         "close_window", &EditorCore::close_active_window,
         "next_window", &EditorCore::next_window,
+        "set_active_window", &EditorCore::set_active_window,
         "undo", &EditorCore::undo,
         "redo", &EditorCore::redo,
         "command_mode", &EditorCore::enter_command_mode,