Add visual-line mode support with tests and UI integration.

- Introduced visual-line mode for multi-line selection and edits.
- Implemented commands, rendering, and keyboard shortcuts.
- Added tests for broadcast operations in visual-line mode.

TerminalInputHandler.h

@@ -30,5 +30,8 @@ private:
 	// Simple meta (ESC) state for ESC sequences like ESC b/f
 	bool esc_meta_ = false;
 
+	// Mouse drag selection state
+	bool mouse_selecting_ = false;
+
 	Editor *ed_ = nullptr; // attached editor for uarg handling
 };