fix(metatype): reject duplicate variant names in define

Two same-named variants in a constructed enum silently succeeded —
construction (.a) and matching would ambiguously pick one. defineEnum
now bails when a variant name repeats, naming it. The name is dynamic so
it sets last_bail_detail directly (bailDetail takes a comptime string);
evalComptimeType renders it as a build-gating diagnostic.

src/ir/interp.zig

@@ -2129,7 +2129,18 @@ pub const Interpreter = struct {
             if (ev.len != 2) return bailDetail("comptime define(): EnumVariant must have `name` and `payload`");
             const vname = ev[0].asString(self) orelse return bailDetail("comptime define(): EnumVariant `name` is not a string");
             const payload_tid = ev[1].asTypeId() orelse return bailDetail("comptime define(): EnumVariant `payload` is not a Type value");
-            fields.append(self.alloc, .{ .name = tbl.internString(vname), .ty = payload_tid }) catch return error.CannotEvalComptime;
+            const vname_id = tbl.internString(vname);
+            // Reject a duplicate variant name loudly — two same-named variants
+            // make construction (`.a`) and matching ambiguous and would silently
+            // pick one. The name is dynamic, so set the bail detail directly
+            // (bailDetail takes a comptime string); evalComptimeType renders it.
+            for (fields.items) |existing| {
+                if (existing.name == vname_id) {
+                    last_bail_detail = std.fmt.allocPrint(self.alloc, "comptime define(): duplicate variant name '{s}'", .{vname}) catch "comptime define(): duplicate variant name";
+                    return error.CannotEvalComptime;
+                }
+            }
+            fields.append(self.alloc, .{ .name = vname_id, .ty = payload_tid }) catch return error.CannotEvalComptime;
         }
 
         // Complete the declared slot IN PLACE: it already has its name + nominal