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_vm.ml

@@ -345,6 +345,13 @@ and run vm =
           let argc = read_u8 frame in
           let name = match consts.(idx) with String s -> s | _ -> "" in
           let args = List.init argc (fun _ -> pop vm) |> List.rev in
+          (* Resolve thunks — the CEK evaluator does this automatically
+             via trampoline, but the VM must do it explicitly before
+             passing args to primitives. *)
+          let args = List.map (fun v ->
+            match v with
+            | Thunk _ -> !Sx_primitives._sx_trampoline_fn v
+            | _ -> v) args in
           let result =
             try
               (* Check primitives FIRST (native implementations of map/filter/etc.),