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

examples/0740-modules-flat-same-name-ufcs-typing/a.sx

@@ -2,5 +2,5 @@
 // a.sx authors `tag` returning a string; imported first → first-wins winner.
 // `show_a`'s `v.tag()` is the caller's OWN author (own == winner → existing UFCS
 // path, byte-for-byte unchanged): typed AND dispatched as a.tag (string).
-tag :: (x: s64) -> string { return "a-string"; }
+tag :: ufcs (x: s64) -> string { return "a-string"; }
 show_a :: () { v : s64 = 10; print("a: v.tag() = {}\n", v.tag()); }