wcc: forrange bindings get a per-loop scope; dup binders reject

The TODO(#11) silent-accept of `for (let (a, a) .. xs)` rested on a
stale premise -- resolvewalk has per-block scopes since #53 and IS
the live w6c_ww checker pass. Bindings now install in a per-LOOP
scope (the N_MCASE pattern), so a duplicate name within one pattern
errs "redeclared in same scope" (cstage parity via stamptuplebinds'
scopedefine-nil check) while sequential same-name loops stay legal.
kwtab restores its stated alphabetical order (`import` before `is`,
both stages, kinds swapped in lockstep with names).

cmd/wcc/tok.c

@@ -24,8 +24,8 @@ static const struct kwent kwtab[] = {
 	{ "fn",       TK_FN       },
 	{ "for",      TK_FOR      },
 	{ "if",       TK_IF       },
-	{ "is",       TK_IS       },
 	{ "import",   TK_USE      },
+	{ "is",       TK_IS       },
 	{ "let",      TK_LET      },
 	{ "match",    TK_MATCH    },
 	{ "nil",      TK_NIL      },