spec: vector primitives — bounds-checked ref/set!, vector-copy start/end slice

vector-ref and vector-set! now raise Eval_error on out-of-bounds index instead of
an OCaml array exception. vector-copy accepts optional start and end parameters for
slicing (R7RS §6.8). spec/primitives.sx doc updated to reflect slice params.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/primitives.sx

@@ -227,9 +227,11 @@
 
 (define-primitive
   "vector-copy"
-  :params ((v :as vector))
+  :params ((v :as vector)
+    (start :as number :optional true)
+    (end :as number :optional true))
   :returns "vector"
-  :doc "Independent shallow copy.")
+  :doc "Shallow copy of vector, optionally sliced from start (inclusive) to end (exclusive).")
 
 (define-primitive
   "min"