feat: add algebraic enums (tagged unions) support

- Generate tagged unions in C backend for enums with data
- Add HirEnumVariant type with fields and namedFields
- Support _Tag and _Data field access in sema
- Support enum variant constants (Result_Ok, Result_Err)
- Support _Data union field access (Ok_0, Err_0, etc.)
- Add algebraic_enums.bux example

Example:
  enum Result {
      Ok(int),
      Err(String)
  }

  let r: Result = Result { tag: Result_Ok };
  r.data.Ok_0 = 42;

examples/algebraic_enums.bux

@@ -0,0 +1,29 @@
+// Algebraic Enums - Enums with data (tagged unions)
+extern func Std_Io_PrintLine(s: String);
+extern func Std_Io_PrintInt(n: int);
+
+enum Result {
+    Ok(int),
+    Err(String)
+}
+
+func Main() -> int {
+    let r1: Result = Result { tag: Result_Ok };
+    r1.data.Ok_0 = 42;
+    
+    let r2: Result = Result { tag: Result_Err };
+    r2.data.Err_0 = "error message";
+    
+    if r1.tag == Result_Ok {
+        Std_Io_PrintLine("r1 is Ok:");
+        Std_Io_PrintInt(r1.data.Ok_0);
+        Std_Io_PrintLine("");
+    }
+    
+    if r2.tag == Result_Err {
+        Std_Io_PrintLine("r2 is Err:");
+        Std_Io_PrintLine(r2.data.Err_0);
+    }
+    
+    return 0;
+}