Step 10c: Vector type + unified reactive model test spec (34 tests)

- Vector of value array added to sx_types.ml (prior commit)
- Vector primitives in sx_primitives.ml (make-vector, vector-ref,
  vector-set!, vector-length, vector->list, list->vector)
- R7RS vector tests
- test-unified-reactive.sx: 34 tests specifying the unified reactive
  model (provide/context/peek/bind replacing signal/deref split).
  All 34 currently fail — implementation next.
- WASM binary rebuilt

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

hosts/ocaml/lib/sx_primitives.ml

@@ -1301,6 +1301,83 @@ let () =
       Nil
     | _ -> Nil);
 
+  (* --- Unified reactive model (Step 10c) ---
+     provide wraps value in a Signal (reactive cell).
+     context unwraps the signal + registers in tracking context.
+     peek unwraps without tracking.
+     provide! mutates the signal and notifies subscribers. *)
+
+  let _tracking_active : bool ref = ref false in
+  let _tracking_deps : value list ref = ref [] in
+
+  register "provide-reactive!" (fun args ->
+    match args with
+    | [String name; value] ->
+      let sig' = { s_value = value; s_subscribers = []; s_deps = [] } in
+      let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+      Hashtbl.replace _scope_stacks name (Signal sig' :: stack); Nil
+    | _ -> raise (Eval_error "provide-reactive!: expected (name value)"));
+
+  register "provide-pop-reactive!" (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);
+
+  register "provide-set!" (fun args ->
+    match args with
+    | [String name; new_value] ->
+      let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+      (match stack with
+       | Signal sig' :: _ ->
+         sig'.s_value <- new_value;
+         List.iter (fun sub -> sub ()) sig'.s_subscribers;
+         Nil
+       | _ -> raise (Eval_error (Printf.sprintf
+           "provide-set!: '%s' is not a reactive provide" name)))
+    | _ -> raise (Eval_error "provide-set!: expected (name new-value)"));
+
+  register "peek" (fun args ->
+    match args with
+    | (String name) :: _ ->
+      let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+      (match stack with
+       | Signal sig' :: _ -> sig'.s_value
+       | v :: _ -> v
+       | [] -> Nil)
+    | _ -> raise (Eval_error "peek: expected (name)"));
+
+  register "tracking-start!" (fun _args ->
+    _tracking_active := true; _tracking_deps := []; Nil);
+
+  register "tracking-stop!" (fun _args ->
+    _tracking_active := false;
+    let deps = !_tracking_deps in
+    _tracking_deps := [];
+    List deps);
+
+  register "tracking-active?" (fun _args ->
+    Bool !_tracking_active);
+
+  (* Override context to be tracking-aware *)
+  Hashtbl.remove primitives "context";
+  register "context" (fun args ->
+    match args with
+    | (String name) :: rest ->
+      let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+      (match stack with
+       | Signal sig' :: _ ->
+         (* Register in tracking context if active *)
+         if !_tracking_active then begin
+           if not (List.memq (Signal sig') !_tracking_deps) then
+             _tracking_deps := Signal sig' :: !_tracking_deps
+         end;
+         sig'.s_value
+       | v :: _ -> v
+       | [] -> (match rest with default_val :: _ -> default_val | [] -> Nil))
+    | _ -> Nil);
+
   (* --- Emit / emitted --- *)
 
   register "scope-emit!" (fun args ->