test(metatype): generic type-fn body local (examples/0624)

Locks the generic-type-fn prelude eval (d87d86d): make_status($T)
assembles a variant list in a local then mints, with the ok payload = T.

examples/0624-comptime-metatype-generic-typefn-local.sx

@@ -0,0 +1,34 @@
+// A GENERIC type-fn (`($T) -> Type`) may use LOCALS before its `return` — the
+// full body is comptime-evaluated, not just the return expression. Here
+// `make_status` assembles a variant list in a local `vs` (whose `ok` payload is
+// the type parameter `T`), then mints `Status` from it via `make_enum`. The
+// equivalent in a non-generic builder already worked (examples/0620); this
+// extends it to the generic case.
+#import "modules/std.sx";
+#import "modules/std/meta.sx";
+
+make_status :: ($T: Type) -> Type {
+    vs := EnumVariant.[
+        EnumVariant.{ name = "ok",      payload = T },      // payload = the type arg
+        EnumVariant.{ name = "pending", payload = void },
+        EnumVariant.{ name = "failed",  payload = i64 },    // error code
+    ];
+    return make_enum("Status", vs);
+}
+
+Status :: make_status(f64);
+
+show :: (s: Status) {
+    if s == {
+        case .ok:      (v) { print("ok={}\n", v); }
+        case .pending:     { print("pending\n"); }
+        case .failed:  (e) { print("failed code={}\n", e); }
+    }
+}
+
+main :: () -> i32 {
+    show(.ok(2.5));
+    show(.pending);
+    show(.failed(404));
+    return 0;
+}

examples/expected/0624-comptime-metatype-generic-typefn-local.exit

@@ -0,0 +1 @@
+0

examples/expected/0624-comptime-metatype-generic-typefn-local.stderr

@@ -0,0 +1 @@
+

examples/expected/0624-comptime-metatype-generic-typefn-local.stdout

@@ -0,0 +1,3 @@
+ok=2.500000
+pending
+failed code=404