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.

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1478;
-def errorcount: i32 = 335;
+def corpuscount: i32 = 1485;
+def errorcount: i32 = 338;
 def compilecount: i32 = 17;
 def runcount: i32 = 191;
-def runexitcount: i32 = 935;
-def nativecount: i32 = 2956;
-def corpushash: str = "2ac2342a80c4148968250bf4ce93852b78cdefb6af96622fa533eea4a68837c6";
+def runexitcount: i32 = 939;
+def nativecount: i32 = 2970;
+def corpushash: str = "69453e64f903a75b8a3f27c79e336ae766b3fb7ecfaac85d13f5d1f0c0dc4f50";
 
 type directive = enum i32 {
 	ERROR = 0,