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

examples/0734-modules-flat-same-name-ufcs-ambiguous/a.sx

@@ -1,2 +1,2 @@
 // a.sx authors `dup` (+1). One of two distinct flat authors of `dup`.
-dup :: (x: s64) -> s64 { return x + 1; }
+dup :: ufcs (x: s64) -> s64 { return x + 1; }

examples/0734-modules-flat-same-name-ufcs-ambiguous/b.sx

@@ -1,3 +1,3 @@
 // b.sx authors its OWN `dup` (+2) — the second distinct flat author. Main
 // imports both and authors neither, so `v.dup()` from main is ambiguous.
-dup :: (x: s64) -> s64 { return x + 2; }
+dup :: ufcs (x: s64) -> s64 { return x + 2; }