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.

lib/ww/syntax/parse.ww

@@ -214,11 +214,37 @@ fn parsetype(p: *parser) *node = {
 			let fpl = p.curline;
 			let fpc = p.curcol;
 			let f = newnode(nkind.N_TFIELD, fpf, fpl, fpc);
-			let fid: str;
-			expectident(p, &fid);
-			f.str = fid;
-			expecttok(p, tkind.TK_COLON, "expected ':' in field");
-			f.lhs = parsetype(p);
+			// Three member forms (cstage parse.c:230-244):
+			//   name: type       — regular field
+			//   struct { ... }   — anonymous embedded struct
+			//   Identifier       — bare dotted-ident embedded type
+			// Embeds carry f.str == "" and the type in f.lhs. The
+			// cstage branches on peek≠':'; this parser has no peek,
+			// so consume the leading ident first and branch on ':'.
+			if (p.curkind == tkind.TK_STRUCT) {
+				f.lhs = parsetype(p);
+			} else {
+				let fid: str;
+				expectident(p, &fid);
+				if (p.curkind == tkind.TK_COLON) {
+					advance(p);
+					f.str = fid;
+					f.lhs = parsetype(p);
+				} else {
+					// Rebuild the dotted TNAME the consumed ident
+					// began (parsetype's TK_IDENT collapse, L280-285).
+					let tn = newnode(nkind.N_TNAME, fpf, fpl, fpc);
+					let acc = fid;
+					for (p.curkind == tkind.TK_DOT) {
+						advance(p);
+						if (p.curkind != tkind.TK_IDENT) { break; };
+						acc = joindotted(acc, p.curtext);
+						advance(p);
+					};
+					tn.str = acc;
+					f.lhs = tn;
+				};
+			};
 			if (fhead == nil) { fhead = f; ftail = f; }
 			else { ftail.next = f; ftail = f; };
 			if (!accepttok(p, tkind.TK_COMMA)) { break; };