test(metatype): lock duplicate-variant-name rejection (examples/1180)

Diagnostics example for the define duplicate-name guard (b2db2c5): two
'value' variants -> build-gating error naming the duplicate, exit 1.

examples/1180-diagnostics-metatype-duplicate-variant.sx

@@ -0,0 +1,17 @@
+// A comptime-constructed enum (declare/define) with two same-named variants is
+// rejected loudly. Two `value` variants would make construction (`.value`) and
+// matching ambiguous — `define` bails naming the duplicate, instead of silently
+// minting a malformed enum that picks one arbitrarily.
+#import "modules/std.sx";
+#import "modules/std/meta.sx";
+
+Bad :: define(declare("Bad"), .enum(.{ variants = .[
+    EnumVariant.{ name = "value",  payload = i64 },
+    EnumVariant.{ name = "closed", payload = void },
+    EnumVariant.{ name = "value",  payload = f64 },   // duplicate name
+] }));
+
+main :: () -> i32 {
+    b : Bad = ---;
+    return 0;
+}

examples/expected/1180-diagnostics-metatype-duplicate-variant.exit

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

examples/expected/1180-diagnostics-metatype-duplicate-variant.stderr

@@ -0,0 +1,13 @@
+error: comptime type construction failed: comptime define(): duplicate variant name 'value'
+  --> examples/1180-diagnostics-metatype-duplicate-variant.sx:8:8
+   |
+ 8 | Bad :: define(declare("Bad"), .enum(.{ variants = .[
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+ 9 |     EnumVariant.{ name = "value",  payload = i64 },
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+10 |     EnumVariant.{ name = "closed", payload = void },
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+11 |     EnumVariant.{ name = "value",  payload = f64 },   // duplicate name
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+12 | ] }));
+   | ^^^^^

examples/expected/1180-diagnostics-metatype-duplicate-variant.stdout

@@ -0,0 +1 @@
+