Complete Python eval removal: epoch protocol, scope consolidation, JIT fixes

Route all rendering through OCaml bridge — render_to_html no longer uses
Python async_eval. Fix register_components to parse &key params and &rest
children from defcomp forms. Remove all dead sx_ref.py imports.

Epoch protocol (prevents pipe desync):
- Every command prefixed with (epoch N), all responses tagged with epoch
- Both sides discard stale-epoch messages — desync structurally impossible
- OCaml main loop discards stale io-responses between commands

Consolidate scope primitives into sx_scope.ml:
- Single source of truth for scope-push!/pop!/peek, collect!/collected,
  emit!/emitted, context, and 12 other scope operations
- Removes duplicate registrations from sx_server.ml (including bugs where
  scope-emit! and clear-collected! were registered twice with different impls)
- Bind scope prims into env so JIT VM finds them via OP_GLOBAL_GET

JIT VM fixes:
- Trampoline thunks before passing args to CALL_PRIM
- as_list resolves thunks via _sx_trampoline_fn
- len handles all value types (Bool, Number, RawHTML, SxExpr, Spread, etc.)

Other fixes:
- ~cssx/tw signature: (tokens) → (&key tokens) to match callers
- Minimal Python evaluator in html.py for sync sx() Jinja function
- Python scope primitive stubs (thread-local) for non-OCaml paths
- Reader macro resolution via OcamlSync instead of sx_ref.py

Tests: 1114 OCaml, 1078 JS, 35 Python regression, 6/6 Playwright SSR

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

hosts/ocaml/lib/sx_primitives.ml

@@ -36,10 +36,11 @@ let as_string = function
   | String s -> s
   | v -> raise (Eval_error ("Expected string, got " ^ type_of v))
 
-let as_list = function
+let rec as_list = function
   | List l -> l
   | ListRef r -> !r
   | Nil -> []
+  | Thunk _ as t -> as_list (!_sx_trampoline_fn t)
   | v -> raise (Eval_error ("Expected list, got " ^ type_of v))
 
 let as_bool = function
@@ -316,8 +317,16 @@ let () =
     | [List l] | [ListRef { contents = l }] -> Number (float_of_int (List.length l))
     | [String s] -> Number (float_of_int (String.length s))
     | [Dict d] -> Number (float_of_int (Hashtbl.length d))
-    | [Nil] -> Number 0.0
-    | _ -> raise (Eval_error "len: 1 arg"));
+    | [Nil] | [Bool false] -> Number 0.0
+    | [Bool true] -> Number 1.0
+    | [Number _] -> Number 1.0
+    | [RawHTML s] -> Number (float_of_int (String.length s))
+    | [SxExpr s] -> Number (float_of_int (String.length s))
+    | [Spread pairs] -> Number (float_of_int (List.length pairs))
+    | [Component _] | [Island _] | [Lambda _] | [NativeFn _]
+    | [Macro _] | [Thunk _] | [Keyword _] | [Symbol _] -> Number 0.0
+    | _ -> raise (Eval_error (Printf.sprintf "len: %d args"
+        (List.length args))));
   register "first" (fun args ->
     match args with
     | [List (x :: _)] | [ListRef { contents = x :: _ }] -> x