Add all 23 GoF design pattern implementations (2026-06-13)

02-structural/composite/FileSystemItem.java

@@ -0,0 +1,12 @@
+package composite;
+
+/**
+ * Component interface — the common contract for BOTH files (leaves)
+ * and directories (composites). Clients work through this interface
+ * and never need to know which they're dealing with.
+ */
+public interface FileSystemItem {
+    String getName();
+    long getSize();          // total size in bytes (recursive for directories)
+    void print(String indent); // display the tree
+}