OCaml bootstrapper Phase 2: HTML renderer, SX server, Python bridge

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

hosts/ocaml/lib/sx_runtime.ml

@@ -35,6 +35,7 @@ let sx_str args =
 (** Convert a value to a list. *)
 let sx_to_list = function
   | List l -> l
+  | ListRef r -> !r
   | Nil -> []
   | v -> raise (Eval_error ("Expected list, got " ^ type_of v))
 
@@ -60,6 +61,7 @@ let sx_apply f args_list =
 let sx_append_b lst item =
   match lst with
   | List items -> List (items @ [item])
+  | ListRef r -> r := !r @ [item]; lst  (* mutate in place, return same ref *)
   | _ -> raise (Eval_error ("append!: expected list, got " ^ type_of lst))
 
 (** Mutable dict-set — set key in dict, return value. *)
@@ -74,7 +76,8 @@ let get_val container key =
   match container, key with
   | Dict d, String k -> dict_get d k
   | Dict d, Keyword k -> dict_get d k
-  | List l, Number n -> (try List.nth l (int_of_float n) with _ -> Nil)
+  | (List l | ListRef { contents = l }), Number n ->
+    (try List.nth l (int_of_float n) with _ -> Nil)
   | _ -> raise (Eval_error ("get: unsupported " ^ type_of container ^ " / " ^ type_of key))
 
 (** Register get as a primitive override — transpiled code calls (get d k). *)
@@ -82,7 +85,12 @@ let () =
   Sx_primitives.register "get" (fun args ->
     match args with
     | [c; k] -> get_val c k
-    | _ -> raise (Eval_error "get: 2 args"))
+    | [c; k; default] ->
+      (try
+        let v = get_val c k in
+        if v = Nil then default else v
+      with _ -> default)
+    | _ -> raise (Eval_error "get: 2-3 args"))
 
 
 (* ====================================================================== *)
@@ -195,8 +203,9 @@ let sx_emit a b = prim_call "emit!" [a; b]
 let sx_emitted a = prim_call "emitted" [a]
 let sx_context a b = prim_call "context" [a; b]
 
-(* Trampoline — evaluate thunks iteratively *)
-let trampoline v = v  (* CEK machine doesn't use tree-walk thunks *)
+(* Trampoline — forward-declared in sx_ref.ml, delegates to CEK eval_expr *)
+(* This is a stub; the real trampoline is wired up in sx_ref.ml after eval_expr is defined *)
+let trampoline v = v
 
 (* Value-returning type predicates — the transpiled code passes these through
    sx_truthy, so they need to return Bool, not OCaml bool. *)