feat: implement NPPM_GETMENUHANDLE for plugin menu access

Returns the main menu or plugin submenu handle based on wParam
(NPPMAINMENU vs NPPPLUGINMENU). ComparePlus uses this for dynamic
menu state management during compare mode.

Phase 1d of issue #100 — ComparePlus macOS port.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Hybrid Machine

macos/platform/nppm_handler.mm

@@ -9,6 +9,7 @@
 #include "lexer_styles.h"
 #include "document_manager.h"
 #include "file_operations.h"
+#include "menu_builder.h"
 #include "string_utils.h"
 #include "Notepad_plus_msgs.h"
 #include "Scintilla.h"
@@ -145,6 +146,15 @@ LRESULT handleNppmMessage(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
 			return TRUE;
 		}
 
+		case NPPM_GETMENUHANDLE:
+		{
+			int menuChoice = static_cast<int>(wParam);
+			if (menuChoice == NPPMAINMENU)
+				return reinterpret_cast<LRESULT>(GetMenu(hWnd));
+			else // NPPPLUGINMENU
+				return reinterpret_cast<LRESULT>(getPluginsMenuHandle());
+		}
+
 		case NPPM_GETPLUGINSCONFIGDIR:
 		{
 			@autoreleasepool {