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>

hosts/ocaml/bin/run_tests.ml

@@ -1159,6 +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; *)
   (* Render adapter for test-render-html.sx *)
   load_module "render.sx" spec_dir;
   load_module "canonical.sx" spec_dir;