R7RS compat library + 45-test suite (27 passing, 17 need lib load fix)

lib/r7rs.sx: guard/with-exception-handler macros, error objects
(make-error-object, error-object?, error-message, error-object-irritants),
R7RS aliases (car/cdr/cadr/null?/pair?/procedure?/boolean=?/symbol->string/
number->string/string->number), string->symbol.

spec/tests/test-r7rs.sx: 9 suites covering call/cc (7), raise (4),
guard (5), with-exception-handler (1), error-objects (4), multi-map (6),
cond=> (4), do-iteration (4), r7rs-aliases (10). 27/44 pass — the 17
failures need r7rs.sx auto-load in the test runner (currently commented
out pending transpiled evaluator hang investigation).

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

lib/r7rs.sx

@@ -0,0 +1,63 @@
+(define make-error-object (fn (message irritants) {:irritants irritants :type "error-object" :message message}))
+
+(define
+  error-object?
+  (fn (x) (and (dict? x) (= (get x "type") "error-object"))))
+
+(define
+  error-message
+  (fn (x) (if (error-object? x) (get x "message") (str x))))
+
+(define
+  error-object-irritants
+  (fn (x) (if (error-object? x) (get x "irritants") (list))))
+
+(defmacro
+  with-exception-handler
+  (handler thunk)
+  (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)
+
+(define cadr (fn (x) (first (rest x))))
+
+(define cddr (fn (x) (rest (rest x))))
+
+(define caar (fn (x) (first (first x))))
+
+(define cdar (fn (x) (rest (first x))))
+
+(define caddr (fn (x) (first (rest (rest x)))))
+
+(define cadddr (fn (x) (first (rest (rest (rest x))))))
+
+(define null? nil?)
+
+(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 symbol->string symbol-name)
+
+(define string->symbol make-symbol)
+
+(define number->string (fn (n) (str n)))
+
+(define
+  string->number
+  (fn (s) (if (string-contains? s ".") (parse-float s) (parse-int s))))