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>

shared/sx/pages.py

@@ -313,10 +313,7 @@ async def _eval_slot(expr: Any, env: dict, ctx: Any) -> str:
         sx_text = _wrap_with_env(expr, env)
         service = ctx.get("_helper_service", "") if isinstance(ctx, dict) else ""
         return await bridge.aser_slot(sx_text, ctx={"_helper_service": service})
-    if os.environ.get("SX_USE_REF") == "1":
-        from .ref.async_eval_ref import async_eval_slot_to_sx
-    else:
-        from .async_eval import async_eval_slot_to_sx
+    from .async_eval import async_eval_slot_to_sx
     return await async_eval_slot_to_sx(expr, env, ctx)