Step 10c: fix capabilities, closure-scope, define-library imports

- Initialize _cek_call_ref in sx_ref.ml — fixes 8 capabilities tests
- Rename test variable 'peek' to 'get-val' — collides with new peek
  special form. Fixes closure-scope-edge test.
- Add import clause handling to define-library — was silently skipping
  (import ...) inside library definitions. Fixes 4 define-library tests.

2767/2768 OCaml (1 pre-existing aser/render-to-sx issue).

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

spec/evaluator.sx

@@ -1986,6 +1986,10 @@
                       (map
                         (fn (s) (if (symbol? s) (symbol-name s) (str s)))
                         (rest decl))))
+                  (= kind "import")
+                  (for-each
+                    (fn (import-set) (bind-import-set import-set lib-env))
+                    (rest decl))
                   (= kind "begin")
                   (set! body-forms (append body-forms (rest decl)))
                   :else nil))))