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.

selfhost/test/smoke.ww

@@ -142,7 +142,11 @@ export fn main() i32 = {
 	if (!ascii.isalpha(122)) { return 16; };	// 'z'
 	if (!ascii.isidstart(95)) { return 17; };	// '_'
 	if (!ascii.isidpart(48)) { return 18; };	// '0' is part
-	if (ascii.digitval(70) != 15) { return 19; };	// 'F' = 15
+	let dv: (i32 | void) = ascii.digitval(70);
+	match (dv) {
+	case let v: i32 => { if (v != 15) { return 19; }; };	// 'F' = 15
+	case void => { return 19; };
+	};
 	if (ascii.tolower(65) != 97) { return 20; };	// 'A' -> 'a'
 
 	// Probe 7 — file open/read via the new os APIs. /proc/self/cmdline