wcc: add the opaque abstract type (kind + UNDEFINED sentinel + name-binding) (#108)

#108 sub-fold (a): TY_OPAQUE exists, is name-bindable, and carries an
UNDEFINED size sentinel. Mirrors the #85 `size` fold pattern at every
site, both stages (rule-10).

opaque is abstract + UNSIZED: prim()'d with size=align=SIZE_UNDEFINED
(NOT 0 — a 0 would let a bare `let x: opaque` fabricate a 0-byte local),
mirroring harec builtin_type_opaque (ref/harec/src/types.c:1446). ww had
no incomplete-size sentinel, so this fold ADDS one: cstage
`#define SIZE_UNDEFINED ((u64)-1)` (== harec types.h:58 (size_t)-1) and
wwstage `def SIZE_UNDEFINED: u64 = 18446744073709551615`.

Legal only behind indirection: `*opaque` (8B ptr) and `[]opaque` (24B
slice header) construct correctly because type_ptr/type_slice (and the
wwstage typeptr/typeslice) size themselves independent of the element.
opaque is deliberately absent from is-int/unsigned/num/float and from
the size-classification switches (let_emit_size / tupleelemslot /
fieldslotsize) on both stages — it only reaches those as TY_PTR/TY_SLICE.

The use-restriction GUARDS (reject bare opaque / size(opaque) / opaque
field / [N]opaque / []opaque-indexing), assignability, and cgen-verify
are the separate sub-folds (b)/(c)/(d) — NOT here.

opaque is unused by the bootstrap, so 990-997 stay byte-identical
(inert, like #85). Regenerates the w6c/wwdump combined.ww (typ.ww +
check.ww embedded). New probe 960_opaque_decl_run exercises `*opaque`
and `[]opaque` (.len/.ptr) behind indirection.

cmd/wcc/check.c

@@ -50,6 +50,7 @@ lookup_builtin(const char *name)
 	if (strcmp(name, "uint") == 0)    return ty_uint;
 	if (strcmp(name, "uintptr") == 0) return ty_uintptr;
 	if (strcmp(name, "size") == 0)    return ty_size;	/* #85 fold-2 */
+	if (strcmp(name, "opaque") == 0)  return ty_opaque;	/* #108(a) */
 	if (strcmp(name, "f32") == 0)     return ty_f32;
 	if (strcmp(name, "f64") == 0)     return ty_f64;
 	if (strcmp(name, "str") == 0)     return ty_str;