Fix r7rs.sx: remove guard macro (transpiler blocker), fix null?/boolean=?

guard macro expansion loops in the transpiled evaluator because
expand_macro→eval_expr→CEK can't handle let+first/rest in macro
bodies. Removed guard macro; will re-add as special form once
transpiler handles runtime AST construction (cons/append/make-symbol).

Fixed null? to handle empty lists (not just nil).
Fixed boolean=? to use = instead of undefined eq?.

2561/2568 tests pass (37 new vs baseline, 5 guard + 2 scope pending).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/run_tests.ml

@@ -1159,8 +1159,8 @@ let run_spec_tests env test_files =
        with e -> Printf.eprintf "Warning: %s: %s\n%!" name (Printexc.to_string e))
     end
   in
-  (* R7RS compatibility library — TODO: debug hang in transpiled eval *)
-  (* load_module "r7rs.sx" lib_dir; *)
+  (* R7RS compatibility library — minimal test version *)
+  load_module "r7rs.sx" lib_dir;
   (* Render adapter for test-render-html.sx *)
   load_module "render.sx" spec_dir;
   load_module "canonical.sx" spec_dir;

lib/r7rs.sx

@@ -18,16 +18,6 @@
   (quasiquote
     (handler-bind (((fn (c) true) (unquote handler))) ((unquote thunk)))))
 
-(defmacro
-  guard
-  (var-and-clauses &rest body)
-  (let
-    ((var (first var-and-clauses)) (clauses (rest var-and-clauses)))
-    (quasiquote
-      (handler-bind
-        (((fn (c) true) (fn ((unquote var)) (cond (splice-unquote clauses) (else (raise (unquote var)))))))
-        (splice-unquote body)))))
-
 (define car first)
 
 (define cdr rest)
@@ -44,13 +34,13 @@
 
 (define cadddr (fn (x) (first (rest (rest (rest x))))))
 
-(define null? nil?)
+(define null? (fn (x) (or (nil? x) (and (list? x) (empty? x)))))
 
 (define pair? (fn (x) (and (list? x) (not (empty? x)))))
 
 (define procedure? (fn (x) (or (lambda? x) (callable? x))))
 
-(define boolean=? (fn (a b) (eq? a b)))
+(define boolean=? (fn (a b) (= a b)))
 
 (define symbol->string symbol-name)