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/ast/ast.hon

@@ -83,6 +83,7 @@ ExprKind :: enum {
     try_expr
     catch_expr
     typed_constant
+    union_constant
     comptime_expr
     literal_void
     literal_undefined
@@ -196,6 +197,7 @@ NodeKind :: enum {
     stmt_errdefer
     stmt_block
     expr_typed_constant
+    expr_union_constant
     expr_comptime
     literal_float_value
     literal_void
@@ -611,6 +613,17 @@ add_typed_constant proc(
     }, at)
 }
 
+#! retains an untagged union's complete native-layout bytes as a string literal.
+add_union_constant proc(
+    builder @mut Builder, at Span, type, bytes NodeId,
+) NodeId ! mem.AllocError {
+    return try add_node(builder, Node{
+        kind = .expr_union_constant,
+        data0 = NodeData{ node_id = type },
+        data1 = NodeData{ node_id = bytes },
+    }, at)
+}
+
 add_comptime_expr proc(builder @mut Builder, at Span, operand NodeId) NodeId ! mem.AllocError {
     return try add_node(builder, Node{
         kind = .expr_comptime,
@@ -1599,6 +1612,7 @@ expression_kind proc(kind NodeKind) ExprKind {
         .expr_try: .try_expr
         .expr_catch: .catch_expr
         .expr_typed_constant: .typed_constant
+        .expr_union_constant: .union_constant
         .expr_comptime: .comptime_expr
         .expr_block: .block_expr
         .literal_float_value: .literal_float
@@ -1652,6 +1666,15 @@ typed_constant proc(tree @Tree, id NodeId) TypedConstantView {
     }
 }
 
+union_constant proc(tree @Tree, id NodeId) TypedConstantView {
+    node :: get_node(&tree.storage, id)
+    debug.assert(node.kind == .expr_union_constant)
+    return TypedConstantView{
+        type = node.data0.node_id,
+        value = node.data1.node_id,
+    }
+}
+
 catch_expr proc(tree @Tree, id NodeId) CatchExprView {
     node :: get_node(&tree.storage, id)
     debug.assert(node.kind == .expr_catch)
@@ -2007,7 +2030,7 @@ node_layout proc(node Node) NodeLayout {
         .literal_int, .literal_float, .literal_float_value, .literal_string, .literal_bool, .literal_null,
         .literal_void, .literal_undefined, .expr_identifier, .invalid_expr: {}
         .expr_unary: layout.direct_children = 2
-        .expr_binary, .expr_range, .expr_index, .expr_selection, .expr_typed_constant,
+        .expr_binary, .expr_range, .expr_index, .expr_selection, .expr_typed_constant, .expr_union_constant,
         .expr_fallible_type, .expr_sum_type, .expr_array_type, .expr_slice_type:
             layout.direct_children = 6
         .expr_pointer_type: layout.direct_children = 4