issue 0153 RESOLVED: pin generic return-type resolution to the fn's defining module

inferGenericReturnType resolved a generic call's return-type AST ($R, !E) in
the CALL-SITE module context. For a re-exported fn the error-set name (LE /
IoErr, re-exported as LE :: lib.LE) resolved through the call-site alias to a
TypeId NOT tagged .error_set, so the planned result was a tuple whose last
field wasn't an error set — errorChannelOf saw a plain tuple and the value-
failable's ! channel was lost (try/or rejected it / built a malformed i1 PHI).

monomorphizeFunction already pins the source to the fn's defining module
before resolving the return type; inferGenericReturnType did not, so the
planned call-result type disagreed with the instance's real signature. Fix:
pin the source to fd.body.source_file around the return-type resolution
(binding-build stays in the call-site context — its args are typed there).

Regression test examples/1058-errors-reexport-value-failable-channel.sx
(+ companion lib.sx). Suite green 732/0.

src/ir/generics.zig

@@ -284,6 +284,18 @@ pub const GenericResolver = struct {
         // tag.
         var scope = TypeBindingScope.enter(self.l, tmp_bindings);
         defer scope.exit();
+        // Resolve the return type in the function's DEFINING module, exactly
+        // as `monomorphizeFunction` does — so a name in the return type (e.g.
+        // the error set of a value-failable `(… , !E)`) resolves to the SAME
+        // TypeId the instance's real signature uses, not whatever a re-export
+        // alias at the call site resolves it to. Without this pin a re-exported
+        // generic value-failable's `!E` resolved to a non-`.error_set` alias,
+        // so the planned call result was a plain tuple and `errorChannelOf`
+        // missed the failable channel (issue 0153). The binding-building above
+        // stays in the call-site context (its args are typed there).
+        const saved_src = self.l.current_source_file;
+        defer self.l.setCurrentSourceFile(saved_src);
+        if (fd.body.source_file) |src| self.l.setCurrentSourceFile(src);
         return self.l.resolveTypeWithBindings(fd.return_type.?);
     }
 };