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

examples/0732-modules-flat-same-name-ufcs/b.sx

@@ -1,4 +1,4 @@
 // b.sx authors its OWN `bump` adding 100. `from_b`'s `v.bump()` must dispatch
 // b.sx's author (+100 → 110), not the first-wins winner from a.sx (+1).
-bump   :: (x: s64) -> s64 { return x + 100; }
+bump   :: ufcs (x: s64) -> s64 { return x + 100; }
 from_b_ufcs :: () -> s64 { v : s64 = 10; return v.bump(); }