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/b.sx

@@ -3,5 +3,5 @@
 // dispatched AND typed as b.tag (s64 = 110), not the first-wins winner from a.sx
 // (string). `print` types each arg from the call plan, so a mistype here boxes
 // the s64 as a string pointer → segfault before the fix.
-tag :: (x: s64) -> s64 { return x + 100; }
+tag :: ufcs (x: s64) -> s64 { return x + 100; }
 show_b :: () { v : s64 = 10; print("b: v.tag() = {}\n", v.tag()); }