Step 5.5 phases 1-2: merge sx_scope into sx_primitives, 4-child define support

Phase 1: Absorb sx_scope.ml (180 lines) into sx_primitives.ml. Scope stacks,
cookies, and trace infrastructure now live alongside other primitives. All 20
scope primitive registrations moved. References updated in sx_server.ml and
sx_browser.ml. sx_scope.ml deleted.

Phase 2: Transpiler handles (define name :effects (...) (fn ...)) forms.
ml-emit-define and ml-emit-define-body detect keyword at position 2 and use
(last expr) instead. Unblocks transpilation of spec/render.sx and
web/adapter-html.sx which use 4-child defines extensively.

2598/2598 tests passing.

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

hosts/ocaml/bin/sx_server.ml

@@ -140,11 +140,10 @@ let io_batch_mode = ref false
 let io_queue : (int * string * value list) list ref = ref []
 let io_counter = ref 0
 
-(* Scope stacks and cookies — all primitives registered in sx_scope.ml.
-   We just reference the shared state for the IO bridge. *)
-(* Sx_scope accessed directly — library is unwrapped *)
-let _request_cookies = Sx_scope.request_cookies
-let _scope_stacks = Sx_scope.scope_stacks
+(* Scope stacks and cookies — primitives registered in sx_primitives.ml.
+   We reference the shared state for the IO bridge. *)
+let _request_cookies = Sx_primitives._request_cookies
+let _scope_stacks = Sx_primitives._scope_stacks
 
 (* ── App config ─────────────────────────────────────────────────────── *)
 (* Populated from __app-config dict after SX files load.                *)