wcc: Hare-style struct embedding (anon + bare-name)

Struct members can be `struct { ... }` (anonymous nested) or a bare
named type, in addition to `name: type`. The inner struct's fields
are promoted to the outer scope with offsets shifted by the embed
base; codegen already keys off Tfield.offset so cgen needs no change.
Errors on non-struct embed or name collision.

test/wcc/700_e2e.c

@@ -344,6 +344,20 @@ static const struct row rows[] = {
 	  "  os.assert(r == 42, \"sum mismatch\\n\");\n"
 	  "  return r: i32;\n"
 	  "};", 42 },
+	/* Hare-style struct embedding: bare-name embed + anonymous
+	 * nested struct, fields promoted to the outer scope. Both
+	 * field-by-field assignment and flat struct-literal init must
+	 * see promoted fields. */
+	{ "type point = struct { x: i32, y: i32 };\n"
+	  "type vec = struct {\n"
+	  "  point,\n"
+	  "  struct { z: i32 },\n"
+	  "  w: i32,\n"
+	  "};\n"
+	  "fn main() i32 = {\n"
+	  "  let v: vec = vec { x = 1, y = 2, z = 3, w = 36 };\n"
+	  "  return v.x + v.y + v.z + v.w;\n"
+	  "};", 42 },
 	/* 3-field tuple destructure in for-range */
 	{ "fn main() i32 = {\n"
 	  "  let buf: [3]i64;\n"