fix(metatype): reject declare() never completed by define()

A bare declare("X") with no define left a zero-field nominal slot that
panicked at codegen (verifySizes: llvm_size != ir_size). evalComptimeType
now detects a zero-variant tagged_union result and emits a clean
build-gating diagnostic naming the type — a zero-variant enum is never a
legitimate construction result (defineEnum rejects empty variant lists
too). Self-reference (a declared slot completed by define) is unaffected.

src/ir/lower/comptime.zig

@@ -473,7 +473,21 @@ pub fn evalComptimeType(self: *Lowering, expr: *const Node) ?TypeId {
         }
         return null;
     };
-    return result.asTypeId();
+    const tid = result.asTypeId() orelse return null;
+    // A bare `declare("X")` that is never completed by a `define(handle, …)`
+    // leaves a zero-FIELD nominal slot (an undefined enum). Sizing / constructing
+    // / emitting it panics at codegen (`verifySizes`: llvm_size != ir_size).
+    // Reject it loudly here — a zero-variant enum is never a legitimate result
+    // (`defineEnum` rejects an empty variant list too).
+    if (!tid.isBuiltin()) {
+        const info = self.module.types.get(tid);
+        if (info == .tagged_union and info.tagged_union.fields.len == 0) {
+            if (self.diagnostics) |d|
+                d.addFmt(.err, expr.span, "type '{s}' is declared but never defined — complete it with define(handle, info)", .{self.module.types.getString(info.tagged_union.name)});
+            return null;
+        }
+    }
+    return tid;
 }
 
 /// Rename a nominal type to a new name, re-keying `intern_map` so