ascii: graduate digitval to (i32 | void); selfhost tagged ABI follow-on

ascii.digitval returns (i32 | void) instead of an i32 -1 sentinel.
Two callers updated to match-on the result (lib/ww/lex/lex.ww escape
parse, selfhost/test/smoke.ww probe 6).

`!` would have been more idiomatic at both call sites — both have
verified isxdigit beforehand — but the selfhost parser doesn't yet
recognize postfix `!`/`?`, so using them in bootstrap-bound code
breaks the 993/995 byte-identity gates. Match is fine for now.

Selfhost cgen follow-on for the 8-byte-rounded tagged-union ABI
(landed in 1e2f55a for the C side):
- cgenutil.slotsize: tagged size = 8 (tag) + max(payload), padded to
  8-byte multiple. Was hardcoded 24.
- cgendecl prologue: spill size/8 arg registers, not always 3.
- cgenstmt cglet tagged-call path: spill the CX value-word only when
  the slot is >16 bytes.

All three were emitting 3-register patterns appropriate to (T | str)
sized unions and overflowing the new 16-byte (i32 | void) slots.

lib/ascii/ascii.ww

@@ -53,9 +53,9 @@ export fn isxdigit(c: rune) bool = {
 	return false;
 };
 
-// digitval — value of `c` as a hex/decimal digit, or -1 if not one.
-// Useful when scanning numeric literals.
-export fn digitval(c: rune) i32 = {
+// digitval — value of `c` as a hex/decimal digit. void variant means
+// `c` isn't a hex digit. Useful when scanning numeric literals.
+export fn digitval(c: rune) (i32 | void) = {
 	if (isdigit(c)) { return (c - 48): i32; };
 	if (c >= 65) {
 		if (c <= 70) { return ((c - 65) + 10): i32; };
@@ -63,7 +63,7 @@ export fn digitval(c: rune) i32 = {
 	if (c >= 97) {
 		if (c <= 102) { return ((c - 97) + 10): i32; };
 	};
-	return -1;
+	return;
 };
 
 // isidstart / isidpart — identifier classes used by the lexer.