feat: materialize address-free compile-time untagged unions

Snapshot full union storage rather than choosing an overlapping field. Preserve structural union source syntax independently of nominal identity so direct literal arguments materialize and share normalized specialization keys. Reject uninitialized bytes and address-bearing member layouts.

source/sema/resolve.hon

@@ -546,6 +546,11 @@ resolve_expr proc(
             try resolve_expr(state, scope, value.type)
             try resolve_expr(state, scope, value.value)
         }
+        .union_constant: {
+            value :: ast.union_constant(tree, id)
+            try resolve_expr(state, scope, value.type)
+            try resolve_expr(state, scope, value.value)
+        }
         .literal_int, .literal_float, .literal_bool, .literal_string, .literal_null, .literal_void,
         .literal_undefined: {}
         .enum_literal: {}