phase: 20.49 COMPLETE; 20.50 Flow+String minimal reps; 20.51 selfhost v0/v1 minimal (Option A/B); hv1-inline binop/unop/copy; docs + run_all + CURRENT_TASK -> 21.0

apps/macros/examples/echo_macro.hako

@@ -0,0 +1,9 @@
+// Example user macro: echo (identity)
+// Contract: expand(json: string) -> string (AST JSON v0)
+
+static box MacroBoxSpec {
+  expand(json) {
+    // In real macros, parse and transform AST JSON here.
+    return json
+  }
+}