sx-http: silent JIT fallback, load signals+engine, fix render-to-html

JIT runtime errors now silently fall back to CEK without disabling the
compiled bytecode or logging. This prevents render-to-html from being
permanently disabled when one page has an unresolved symbol (e.g. the
homepage stepper's <home).

Load spec/signals.sx and web/engine.sx for reactive primitives.
Skip test files and non-rendering directories (tests/, plans/, essays/).

Performance with JIT active (warm, single process, 2MB RSS):
- Aser (component expansion): 21-87ms — faster than Quart baseline
- SSR + shell: variable due to homepage <home fallback issue
- Geography total: ~500ms when JIT stays active

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

hosts/ocaml/lib/sx_vm.ml

@@ -164,10 +164,8 @@ and vm_call vm f args =
        (* Cached bytecode — run on VM using the closure's captured env,
           not the caller's globals. Closure vars were merged at compile time. *)
        (try push vm (call_closure cl args cl.vm_env_ref)
-        with e ->
-          let msg = match e with Eval_error m -> m | e -> Printexc.to_string e in
-          Printf.eprintf "[vm] JIT call failed for %s: %s — falling back to CEK\n%!"
-            (match l.l_name with Some n -> n | None -> "<anon>") msg;
+        with _e ->
+          (* Silent fallback to CEK — error is data-dependent, not a JIT bug *)
           push vm (Sx_ref.cek_call f (List args)))
      | Some _ ->
        (* Compile failed — CEK *)
@@ -181,11 +179,8 @@ and vm_call vm f args =
          | Some cl ->
            l.l_compiled <- Some cl;
            (try push vm (call_closure cl args cl.vm_env_ref)
-            with e ->
-              let msg = match e with Eval_error m -> m | e -> Printexc.to_string e in
-              Printf.eprintf "[vm] JIT first-call failed for %s: %s — marking failed, falling back to CEK\n%!"
-                (match l.l_name with Some n -> n | None -> "<anon>") msg;
-              l.l_compiled <- Some jit_failed_sentinel;
+            with _e ->
+              (* Don't mark failed — error may be data-dependent *)
               push vm (Sx_ref.cek_call f (List args)))
          | None ->
            push vm (Sx_ref.cek_call f (List args))