Step 10b: capability-based sandboxing

Capability primitives promoted from mcp_tree.ml to sx_primitives.ml:
- with-capabilities — push cap set, eval body, restore on exit/error
- current-capabilities — returns active capability list (nil = unrestricted)
- has-capability? — check if capability granted (true when unrestricted)
- require-capability! — raise if capability missing
- capability-restricted? — check if any restrictions active

Infrastructure: _cek_call_ref in sx_types.ml (forward ref pattern)
allows primitives to invoke the CEK evaluator without dependency cycles.

10 new tests: unrestricted defaults, scoping, nesting, restore-on-exit.
2693 total tests, 0 regressions.

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

hosts/ocaml/lib/sx_ref.ml

@@ -818,6 +818,9 @@ let () = trampoline_fn := (fun v ->
 (* Wire up the primitives trampoline so call_any in HO forms resolves Thunks *)
 let () = Sx_primitives._sx_trampoline_fn := !trampoline_fn
 
+(* Wire up CEK call ref so primitives (e.g. with-capabilities) can invoke the evaluator *)
+let () = Sx_types._cek_call_ref := cek_call
+
 (* Override recursive cek_run with iterative loop.
    On error, capture the kont from the last state for comp-trace. *)
 let cek_run_iterative state =