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/bin/sx_server.ml

@@ -696,14 +696,9 @@ let register_jit_hook env =
     | Lambda l ->
       (match l.l_compiled with
        | Some cl when not (Sx_vm.is_jit_failed cl) ->
-         (* Cached bytecode — run on VM, fall back to CEK on runtime error.
-            Mark as failed so we don't retry on every call. *)
+         (* Cached bytecode — run on VM, fall back to CEK on runtime error. *)
          (try Some (Sx_vm.call_closure cl args cl.vm_env_ref)
-          with e ->
-            let fn_name = match l.l_name with Some n -> n | None -> "?" in
-            Printf.eprintf "[jit] DISABLED %s — %s\n%!" fn_name (Printexc.to_string e);
-            l.l_compiled <- Some Sx_vm.jit_failed_sentinel;
-            None)
+          with _e -> None)  (* silent fallback — no disable, no log *)
        | Some _ -> None  (* compile failed or disabled — CEK handles *)
        | None ->
          let fn_name = match l.l_name with Some n -> n | None -> "?" in
@@ -720,10 +715,7 @@ let register_jit_hook env =
            | Some cl ->
              l.l_compiled <- Some cl;
              (try Some (Sx_vm.call_closure cl args cl.vm_env_ref)
-              with e ->
-                Printf.eprintf "[jit] DISABLED %s — %s\n%!" fn_name (Printexc.to_string e);
-                l.l_compiled <- Some Sx_vm.jit_failed_sentinel;
-                None)
+              with _e -> None)  (* silent fallback, keep bytecode *)
            | None -> None
          end)
     | _ -> None)