feat: emit NPPN_BUFFERACTIVATED, BEFORESHUTDOWN, TBMODIFICATION

- NPPN_BUFFERACTIVATED: fired from switchToTabInView() with buffer ID
  in idFrom, so plugins can track which buffer is now active
- NPPN_BEFORESHUTDOWN: fired before NPPN_SHUTDOWN for last-chance cleanup
- NPPN_TBMODIFICATION: fired after NPPN_READY to signal toolbar readiness

These notifications are required by ComparePlus for compare-pair
bookkeeping, resource cleanup, and toolbar icon registration.

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

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

Author: Hybrid Machine

macos/platform/app_delegate.mm

@@ -568,6 +568,14 @@ - (void)applicationDidFinishLaunching:(NSNotification*)notification
 		pluginManager().notify(&readyNotif);
 	}
 
+	// Signal plugins that toolbar is ready for icon registration
+	{
+		SCNotification tbNotif{};
+		tbNotif.nmhdr.hwndFrom = ctx().mainHwnd;
+		tbNotif.nmhdr.code = NPPN_TBMODIFICATION;
+		pluginManager().notify(&tbNotif);
+	}
+
 	NSLog(@"=== Notepad++ macOS Port — Phase 7 ===");
 	NSLog(@"Settings, split view, edit commands, encoding, session, drag-and-drop!");
 }
@@ -643,6 +651,14 @@ - (BOOL)applicationShouldTerminateAfterLastWindowClosed:(NSApplication*)sender
 
 - (void)applicationWillTerminate:(NSNotification*)notification
 {
+	// Notify plugins that we are about to shut down (last chance for cleanup)
+	{
+		SCNotification beforeNotif{};
+		beforeNotif.nmhdr.hwndFrom = ctx().mainHwnd;
+		beforeNotif.nmhdr.code = NPPN_BEFORESHUTDOWN;
+		pluginManager().notify(&beforeNotif);
+	}
+
 	// Notify plugins that we are shutting down
 	{
 		SCNotification shutdownNotif{};

macos/platform/document_manager.mm

@@ -148,6 +148,15 @@ void switchToTabInView(int viewIndex, int tabIndex)
 	NSString* title = WideToNSString(doc.title.c_str());
 	[ctx().mainWindow setTitle:[NSString stringWithFormat:@"Notepad++ — %@", title]];
 	updateWindowDocumentEdited();
+
+	// Notify plugins that a buffer was activated
+	{
+		SCNotification notif{};
+		notif.nmhdr.hwndFrom = ctx().mainHwnd;
+		notif.nmhdr.code = NPPN_BUFFERACTIVATED;
+		notif.nmhdr.idFrom = static_cast<uintptr_t>(doc.bufferId);
+		pluginManager().notify(&notif);
+	}
 }
 
 void switchToTab(int tabIndex)