Unify scope mechanism: one world (hashtable stacks everywhere)

Replace continuation-based scope frames with hashtable stacks for all
scope operations. The CEK evaluator's scope/provide/context/emit!/emitted
now use scope-push!/pop!/peek/emit! primitives (registered in
sx_primitives table) instead of walking continuation frames.

This eliminates the two-world problem where the aser used hashtable
stacks (scope-push!/pop!) but eval-expr used continuation frames
(ScopeFrame/ScopeAccFrame). Now both paths share the same mechanism.

Benefits:
- scope/context works inside eval-expr calls (e.g. (str ... (context x)))
- O(1) scope lookup vs O(n) continuation walking
- Simpler — no ScopeFrame/ScopeAccFrame/ProvideFrame creation/dispatch
- VM-compiled code and CEK code both see the same scope state

Also registers scope-push!/pop!/peek/emit!/collect!/collected/
clear-collected! as real primitives (sx_primitives table) so the
transpiled evaluator can call them directly.

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

hosts/ocaml/bin/sx_server.ml

@@ -129,6 +129,20 @@ let io_counter = ref 0
     scope-push!/pop!) and step-sf-context (via get-primitive "scope-peek"). *)
 let _scope_stacks : (string, value list) Hashtbl.t = Hashtbl.create 8
 
+let () = Sx_primitives.register "scope-push!" (fun args ->
+  match args with
+  | [String name; value] ->
+    let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+    Hashtbl.replace _scope_stacks name (value :: stack); Nil
+  | _ -> Nil)
+
+let () = Sx_primitives.register "scope-pop!" (fun args ->
+  match args with
+  | [String name] ->
+    let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+    (match stack with _ :: rest -> Hashtbl.replace _scope_stacks name rest | [] -> ()); Nil
+  | _ -> Nil)
+
 let () = Sx_primitives.register "scope-peek" (fun args ->
   match args with
   | [String name] ->
@@ -160,6 +174,22 @@ let () = Sx_primitives.register "collected" (fun args ->
     (match stack with List items :: _ -> List items | _ -> List [])
   | _ -> List [])
 
+let () = Sx_primitives.register "scope-emit!" (fun args ->
+  match args with
+  | [String name; value] ->
+    let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+    (match stack with
+     | List items :: rest ->
+       Hashtbl.replace _scope_stacks name (List (items @ [value]) :: rest)
+     | Nil :: rest ->
+       Hashtbl.replace _scope_stacks name (List [value] :: rest)
+     | [] ->
+       (* Lazy root scope *)
+       Hashtbl.replace _scope_stacks name [List [value]]
+     | _ :: _ -> ());
+    Nil
+  | _ -> Nil)
+
 let () = Sx_primitives.register "clear-collected!" (fun args ->
   match args with
   | [String name] ->