btdu.state: Reset compareRoot during rebuild in compare mode

Reset the compareRoot alongside browserRoot when starting a rebuild in
compare mode. This ensures all browser path sample data is properly
cleared for both the main and comparison trees.

Author: CyberShadow

source/btdu/state.d

@@ -648,6 +648,8 @@ void startRebuild()
 
 	// Reset all BrowserPath sample data and sharing group links
 	browserRoot.reset();
+	if (compareMode)
+		compareRoot.reset();
 	markTotalSamples = 0;
 
 	// Clear all pathData[i].next pointers to avoid stale values causing cycles