selfhost: add @[Drop] attribute for user-defined structs

- ast.bux: add isDrop field to Decl
- parser.bux: parse @[Drop] attribute and attach to struct Decls
- hir_lower.bux: auto-drop now looks for TypeName_Drop for any type
- Test: _test_drop_user verifies Buffer_Drop is auto-called

src/hir_lower.bux

@@ -2164,6 +2164,12 @@ func Lcx_BuildAutoDropFree(ctx: *LowerCtx, typeName: String) -> String {
             return String_Concat(String_Concat("Map_Drop_", kType), String_Concat("_", vType));
         }
     }
+    // User-defined types with @[Drop]: look for TypeName_Drop function
+    let dropName: String = String_Concat(typeName, "_Drop");
+    let dropSym: Symbol = Scope_Lookup(ctx.scope, dropName);
+    if dropSym.decl != null as *Decl {
+        return dropName;
+    }
     return "";
 }