ww: add Hare-style is/as postfix ops on tagged unions

`e is T` returns bool (variant tag == T's index); `e as T` unwraps
to T or exit(1) on mismatch. Postfix, same precedence as `:` cast.
TK_IS / N_TYPETEST / N_TYPEASSERT appended at the tail of their
enums so every prior numeric value stays unchanged — the
990_selfhost wwdump-diff stays byte-clean.

Cgen mirrors the match-case slot-based load (tag at +0, value at
+8/+16), so an N_IDENT tagged-union local works just like a
match scrutinee. Selfhost cgen inlines the slot resolution
because the wwstage cgen drops sign bits on `*i32` output
parameters in this position.

Renames `errors.is` -> `errors.equal` (the only naming collision;
the existing comment already noted it shared shape with
strings.equal/bytes.equal).

lib/ww/parse/expr.ww

@@ -306,6 +306,26 @@ fn parsepostfix(p: *parser, lhs: *node) *node = {
 			cur = n;
 			continue;
 		};
+		// Hare-style postfix:
+		//   `e as T` — assert lhs is variant T (abort otherwise) → T
+		//   `e is T` — bool: does lhs currently hold variant T?
+		// Same precedence level as the `:` cast.
+		if (p.curkind == TK_AS) {
+			advance(p);
+			let n: *node = newnode(p.a, N_TYPEASSERT, pf, pl, pc);
+			n.lhs = cur;
+			n.rhs = parsetype(p);
+			cur = n;
+			continue;
+		};
+		if (p.curkind == TK_IS) {
+			advance(p);
+			let n: *node = newnode(p.a, N_TYPETEST, pf, pl, pc);
+			n.lhs = cur;
+			n.rhs = parsetype(p);
+			cur = n;
+			continue;
+		};
 		break;
 	};
 	return cur;