test(metatype): lock declare()-never-defined rejection (examples/1181)

Diagnostics example for the bare-declare guard (14f30f3): an unfinished
declare("Undef") -> build-gating error naming the type, exit 1.

examples/1181-diagnostics-metatype-declare-never-defined.sx

@@ -0,0 +1,14 @@
+// A `declare("X")` that is never completed by `define(handle, info)` is rejected
+// loudly. `declare` mints an empty (undefined) nominal slot; without a matching
+// `define` it has zero variants, which is never a usable type — sizing /
+// constructing it would otherwise panic at codegen. The diagnostic names the
+// type and points at the bare `declare` so the missing `define` is obvious.
+#import "modules/std.sx";
+#import "modules/std/meta.sx";
+
+Undef :: declare("Undef");          // never define()d
+
+main :: () -> i32 {
+    x : Undef = ---;
+    return 0;
+}

examples/expected/1181-diagnostics-metatype-declare-never-defined.exit

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

examples/expected/1181-diagnostics-metatype-declare-never-defined.stderr

@@ -0,0 +1,5 @@
+error: type 'Undef' is declared but never defined — complete it with define(handle, info)
+  --> examples/1181-diagnostics-metatype-declare-never-defined.sx:9:10
+   |
+ 9 | Undef :: declare("Undef");          // never define()d
+   |          ^^^^^^^^^^^^^^^^

examples/expected/1181-diagnostics-metatype-declare-never-defined.stdout

@@ -0,0 +1 @@
+