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

examples/0039-basic-free-fn-ufcs-pointer-receiver.sx

@@ -10,9 +10,9 @@
 Counter :: struct { n: s32; }
 
 // FREE functions (defined outside the struct), pointer first param.
-bump :: (c: *Counter) -> s32 { c.n += 1; return c.n; }
+bump :: ufcs (c: *Counter) -> s32 { c.n += 1; return c.n; }
 // reached ONLY via UFCS — must still be emitted.
-reset :: (c: *Counter) { c.n = 0; }
+reset :: ufcs (c: *Counter) { c.n = 0; }
 
 main :: () -> s32 {
     c := Counter.{ n = 10 };