Delete sx_ref.py — OCaml is the sole SX evaluator

Removes the 5993-line bootstrapped Python evaluator (sx_ref.py) and all
code that depended on it exclusively. Both bootstrappers (JS + OCaml)
now use a new synchronous OCaml bridge (ocaml_sync.py) to run the
transpiler. JS build produces identical output; OCaml bootstrap produces
byte-identical sx_ref.ml.

Key changes:
- New shared/sx/ocaml_sync.py: sync subprocess bridge to sx_server.exe
- hosts/javascript/bootstrap.py: serialize defines → temp file → OCaml eval
- hosts/ocaml/bootstrap.py: same pattern for OCaml transpiler
- shared/sx/{html,async_eval,resolver,jinja_bridge,handlers,pages,deps,helpers}:
  stub or remove sx_ref imports; runtime uses OCaml bridge (SX_USE_OCAML=1)
- sx/sxc/pages: parse defpage/defhandler from AST instead of Python eval
- hosts/ocaml/lib/sx_primitives.ml: append handles non-list 2nd arg per spec
- Deleted: sx_ref.py, async_eval_ref.py, 6 Python test runners, misc ref/ files

Test results: JS 1078/1078, OCaml 1114/1114.
sx_docs SSR has pre-existing rendering issues to investigate separately.

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

hosts/ocaml/lib/sx_primitives.ml

@@ -353,8 +353,16 @@ let () =
     | [x; Nil] -> List [x]
     | _ -> raise (Eval_error "cons: value and list"));
   register "append" (fun args ->
-    let all = List.concat_map (fun a -> as_list a) args in
-    List all);
+    match args with
+    | [List la | ListRef { contents = la }; List lb | ListRef { contents = lb }] ->
+      List (la @ lb)
+    | [List la | ListRef { contents = la }; Nil] -> List la
+    | [Nil; List lb | ListRef { contents = lb }] -> List lb
+    | [List la | ListRef { contents = la }; v] -> List (la @ [v])
+    | [v; List lb | ListRef { contents = lb }] -> List ([v] @ lb)
+    | _ ->
+      let all = List.concat_map as_list args in
+      List all);
   register "reverse" (fun args ->
     match args with
     | [List l] | [ListRef { contents = l }] -> List (List.rev l)