wcc/ww: .wwi export-data producer + check_exported_type (#22 M2)

New `w6c -I <out.wwi>` flag (both stages) writes a re-parseable
ww-prototype rendering of a package's EXPORTED surface. M2 dead-code:
nothing consumes .wwi yet (combined.ww stays the live path); the flag is
off on every existing invocation, so the 990-997 byte-id gates and all
prior tests are unperturbed.

The unparse walks the AST type-expr subtree (N_T* nodes), not the
tinfo Type* (which collapses nominal pkg.Name identity). Deterministic
output: package line, byte-sorted imports, byte-sorted decls — a pure
function of the exported API. cmd/w6c/wwi.c + selfhost/cmd/wcc/wwi.ww
emit byte-identical .wwi (new cross-stage byte-id substrate, rule 10).

check_exported_type (drew) rides the producer entry, flag-gated: an
exported signature naming a non-exported nominal is loud-rejected before
any byte is written, identically on both stages. Ports harec
check.c:4092-4168, recursing the type-AST and gating on the resolved
SK_TYPE sym's decl export flag (Sym.exported is vestigial in both
stages; the predeclared synthetic `nomem` decl carries no source
position and is treated as a builtin leaf — cstage parity).

Two wwstage checker AST-mutations are normalized to cstage's pristine
view for byte-id: the N_TPARAM tuple-element wrapper (unwrapped) and the
variadic `T...`→`[]T` param desugar (peeled).

Gate 989_m2wwi_run: ascii/strings/getopt each produce a .wwi that
re-parses (wwdump -a) and is cs==ww byte-identical; a private-type-leak
fixture is rejected identically by both stages (non-vacuous check).

cmd/w6c/gc.h

@@ -56,6 +56,11 @@ void	emit(Cg*, Prog*);
 /* txt.c */
 void	txt_emit(FILE*, Prog *head);
 
+/* wwi.c — `.wwi` export-data producer (w6c -I). M2 dead-code: writes a
+ * re-parseable ww-prototype rendering of the package's exported surface.
+ * Returns non-zero if check_exported_type rejects a dangling export. */
+int	wwi_emit(Checker *c, FILE *of, Node *file);
+
 /* swt.c, peep.c, reg.c — placeholders for now */
 void	peephole(Cg*);
 void	regalloc_init(Cg*);