selfhost: alias-aware istaggedtype for nested-union match

`type error = !(invalid | overflow)` miscompiled — istaggedtype
only matched N_TTAGGED directly, so an `e: error` param spilled
as 8B scalar and the match's slot+8 read trailed into saved BP.

Mirror isstrtype's alias+bang unwrap; add resolvetagged() for
is/as/match sites that need the inner N_TTAGGED. Frame scan
counts via slotsize so wwstage stays byte-identical to cstage.
Unblocks lib/strconv.strerror.

lib/strconv/strconv.ww

@@ -8,6 +8,7 @@
 // today). Graduate to the static-buffer shape once that lands.
 
 use os;
+use strings;
 
 // invalid — input wasn't a valid number in the requested format.
 // Payload is the byte index of the first offending position.
@@ -352,8 +353,13 @@ export fn f64tos(v: f64) str = {
 	return r;
 };
 
-// strerror — Hare has strconv::strerror; ww doesn't ship it yet
-// because a `match (e) { case invalid => ... }` arm over the wider
-// `error = !(invalid | overflow)` union exposes a cstage-vs-wwstage
-// cgen divergence (one cgen spills the unused payload slot, the
-// other elides it). Restore once the cgens converge.
+// strerror — convert an strconv error to a user-readable string.
+// Returns owned str; release via os.free. Mirrors Hare's
+// strconv::strerror.
+export fn strerror(e: error) str = {
+	match (e) {
+	case let v: invalid  => return strings.dup("input is not a valid number");
+	case let v: overflow => return strings.dup("input number doesn't fit target type");
+	};
+	return strings.dup("");
+};