fix(selfhost/c_backend): emit _Tag + struct wrapper for all enums

Simple enums were emitted as plain C enums, but the rest of the codegen
(and source code) expected them to have a .tag field like algebraic enums.
This caused C compilation errors:
- 'request for member tag in something not a structure or union'
- 'field name not in record or union initializer'

Fix: always emit the _Tag typedef + struct wrapper for every enum.
For simple enums, the struct only contains the tag (no _Data union).
For algebraic enums, keep the existing _Data union + struct behavior.

This matches the bootstrap compiler's enum codegen.

src/c_backend.bux

@@ -778,27 +778,7 @@ func CBackend_Generate(mod: *HirModule) -> String {
             }
             vi = vi + 1;
         }
-        if !hasData {
+        // Emit tag enum for all enums (simple and algebraic)
-            // Simple enum: typedef enum { A, B } Name;
-            StringBuilder_Append(&cbe.sb, "typedef enum {\n");
-            vi = 0;
-            while vi < en.variantCount {
-                StringBuilder_Append(&cbe.sb, "    ");
-                StringBuilder_Append(&cbe.sb, en.name);
-                StringBuilder_Append(&cbe.sb, "_");
-                StringBuilder_Append(&cbe.sb, en.variants[vi].name);
-                if vi < en.variantCount - 1 {
-                    StringBuilder_Append(&cbe.sb, ",");
-                }
-                StringBuilder_Append(&cbe.sb, "\n");
-                vi = vi + 1;
-            }
-            StringBuilder_Append(&cbe.sb, "} ");
-            StringBuilder_Append(&cbe.sb, en.name);
-            StringBuilder_Append(&cbe.sb, ";\n\n");
-        } else {
-            // Algebraic enum: tag enum + data union + struct
-            // 1. Tag enum
         StringBuilder_Append(&cbe.sb, "typedef enum {\n");
         vi = 0;
         while vi < en.variantCount {
@@ -815,7 +795,19 @@ func CBackend_Generate(mod: *HirModule) -> String {
         StringBuilder_Append(&cbe.sb, "} ");
         StringBuilder_Append(&cbe.sb, en.name);
         StringBuilder_Append(&cbe.sb, "_Tag;\n\n");
-            // 2. Data union
+
+        if !hasData {
+            // Simple enum: struct wrapper with only tag
+            StringBuilder_Append(&cbe.sb, "typedef struct {\n");
+            StringBuilder_Append(&cbe.sb, "    ");
+            StringBuilder_Append(&cbe.sb, en.name);
+            StringBuilder_Append(&cbe.sb, "_Tag tag;\n");
+            StringBuilder_Append(&cbe.sb, "} ");
+            StringBuilder_Append(&cbe.sb, en.name);
+            StringBuilder_Append(&cbe.sb, ";\n\n");
+        } else {
+            // Algebraic enum: data union + struct (tag enum already emitted above)
+            // 1. Data union
             StringBuilder_Append(&cbe.sb, "typedef union {\n");
             vi = 0;
             while vi < en.variantCount {