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).

cmd/wcc/ww.h

@@ -111,7 +111,7 @@ typedef enum {
 	TK_NIL,
 	TK_TRUE,
 	TK_FALSE,
-	TK_AS,		/* reserved for future cast spelling, not active */
+	TK_AS,		/* Hare-style type assertion: e as T */
 	TK_STATIC,	/* Hare-style storage-class qualifier */
 	TK_MATCH,	/* match expression head */
 	TK_CONST,	/* const binding */
@@ -172,6 +172,11 @@ typedef enum {
 	TK_ARROW,	/* -> (reserved) */
 	TK_FATARROW,	/* => (match arms) */
 
+	/* Appended after TK_FATARROW (not grouped with the keyword block)
+	 * to keep the numeric value of every existing kind unchanged —
+	 * the selfhost wwdump-diff test (990) is byte-sensitive. */
+	TK_IS,		/* Hare-style type test: e is T */
+
 	TK_LAST		/* sentinel for tables */
 } Tkind;
 
@@ -284,6 +289,12 @@ typedef enum {
 	N_MLET,		/* let a, b = expr;  list = N_LET stubs (str, lhs=type), rhs = expr */
 	N_MASSIGN,	/* a, b = expr;  list = lvalue exprs, rhs = expr */
 
+	/* Appended after N_MASSIGN — keeps the numeric value of every
+	 * existing kind unchanged, so the selfhost AST dump still diffs
+	 * byte-for-byte. lhs=value, rhs=variant type expr. */
+	N_TYPETEST,	/* lhs is T  →  bool */
+	N_TYPEASSERT,	/* lhs as T  →  T (abort if tag mismatch) */
+
 	N_LAST
 } Nkind;