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.

test/wcc/700_e2e.c

@@ -470,6 +470,24 @@ static const struct row rows[] = {
 	  "  };\n"
 	  "  return 0;\n"
 	  "};", 40 },
+	/* named alias over `!(A | B)`: param spill must size to the
+	 * flattened union (8B tag + 8B payload), not 8B-scalar. Pinned
+	 * the cstage/wwstage divergence where wwstage's istaggedtype
+	 * was alias-blind and the slot+8 read trailed into saved BP. */
+	{ "type invalid  = !i32;\n"
+	  "type overflow = !void;\n"
+	  "type error    = !(invalid | overflow);\n"
+	  "fn errcode(e: error) i32 = {\n"
+	  "  match (e) {\n"
+	  "    case let v: invalid  => return v: i32;\n"
+	  "    case let v: overflow => return 99;\n"
+	  "  };\n"
+	  "  return 0;\n"
+	  "};\n"
+	  "fn main() i32 = {\n"
+	  "  let e: error = 7: invalid;\n"
+	  "  return errcode(e);\n"
+	  "};", 7 },
 	/* str-typed variant payload: let-init with a string literal,
 	 * match-binding loads ptr+len from the slot */
 	{ "fn main() i32 = {\n"