wwstage: port struct embedding; graduate #59.13

The last frontend-gap pin: wwstage had no Hare struct embedding
(struct { hash.hash, ... }), rejecting lib/crypto/sha256 at parse.

- parse.ww: the three member forms (named / anonymous struct / bare
  dotted-ident embed), consume-then-branch since this parser has no
  peek; embeds carry f.str == "" and the type in f.lhs.
- check.ww N_TSTRUCT flatten: promote the inner struct's flattened
  fields at base+src.offset (check.c:961-990); the embed is one
  nested-struct unit in the slot ladder; the resolved inner AST is
  planted on the TFIELD rhs for cgen.
- check.ww walkers: astoffset / exprtype N_DOT / #251 struct-lit
  field lookups descend embeds through shared helpers; the collision
  and non-struct-embed rejects live in validatestructfields (the
  once-per-decl diagnostic site).
- cgenutil.ww registerstruct: regfieldrun walks the AST against the
  flattened tfield cursor, descending embeds via the planted inner
  AST so promoted fieldinfo entries keep the inner field's own name
  and type node.
- wwi printers unchanged (both stages already emit nameless fields).

sha256_test compiles byte-identically end to end and its 6 tests
pass; 989_lib_byteid is now 44 id / 0 divergent / 0 wwreject.
Fixtures r5913_* (promoted rw, offset shift, anonymous embed,
two-level embed + promoted fn-ptr callee, three rejects); corpus pin
1485/2970.

test/wcc/989_lib_byteid.c

@@ -193,8 +193,12 @@ static const struct ent ents[] = {
 	{ .fixture = "lib/ww/syntax/asttest.ww", .inc = "lib/ww",
 	  .mode = M_ID, .cite = "#59.12 graduated by #146 str==" },
 	/* -------- wwstage front-end gaps (task #59) -------------- */
+	/* #59.13 graduated: wwstage struct embedding (parser three member
+	 * forms, checker tinfo flatten + promoted-name walkers, cgen
+	 * registerstruct embed descend) — the hash.hash embed compiles
+	 * byte-identically end to end. Runtime pins: r5913_*. */
 	{ .fixture = "lib/crypto/sha256/sha256_test.ww",
-	  .mode = M_WWREJECT, .cite = "#59.13" },
+	  .mode = M_ID, .cite = "#59.13 graduated by the embed port" },
 	/* #59.14 graduated to M_ID by #27: the #27b enum-as-int cgen fix
 	 * (value passthrough, not a tagged assertion) + the fnmatch.ww:126
 	 * `'\\': u8` explicit cast (the rune-lit→u8 narrow w6c_ww rejects at