lang: opt-in UFCS — ufcs-marked fns + alias dot-dispatch, generic binding via receiver; one binding builder for plan-side generic returns

src/parser.zig

@@ -266,11 +266,18 @@ pub const Parser = struct {
             return self.parseUnionDecl(name, start_pos, name_is_raw);
         }
 
-        // UFCS alias: name :: ufcs target;
+        // UFCS forms:
+        //   name :: ufcs (params) -> ret { body }   — fn declared dot-callable
+        //   name :: ufcs target;                    — dot-callable alias
         if (self.current.tag == .kw_ufcs) {
             self.advance();
+            if (self.current.tag == .l_paren) {
+                const node = try self.parseFnDecl(name, name_span, name_is_raw, start_pos);
+                node.data.fn_decl.is_ufcs = true;
+                return node;
+            }
             if (self.current.tag != .identifier) {
-                return self.fail("expected function name after 'ufcs'");
+                return self.fail("expected '(' (a ufcs function declaration) or a function name (a ufcs alias) after 'ufcs'");
             }
             const target = self.tokenSlice(self.current);
             self.advance();