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

src/ast.zig

@@ -142,6 +142,11 @@ pub const FnDecl = struct {
     /// is a REQUIRED parameter, so a parser site cannot drop it; the default
     /// here serves only post-check synthesized decls (which are never raw).
     is_raw: bool = false,
+    /// `name :: ufcs (params) { body }` — the fn opted into dot-call
+    /// dispatch (`recv.name(args)`). Dot-calls on free functions are
+    /// OPT-IN: only `is_ufcs` fns and `ufcs` aliases dispatch; a plain
+    /// fn is callable directly or via `|>` only.
+    is_ufcs: bool = false,
 };
 
 pub const Param = struct {