host: malformed posts degrade instead of 502 (parse-safe + 500 boundary)

A post whose sx_content is malformed SX (e.g. "<h1 ...)" — a typo'd paren)
made GET /<slug>/ return 502, surfaced as a Cloudflare error page. Root
cause: the kernel `parse` raises a native Parse_error that an SX (guard ...)
cannot catch (guard only traps SX conditions), so host/blog-render's guard
around (parse sx) was ineffective; the exception escaped to the http-listen
loop, which swallowed it and wrote NO response — a dropped connection that
Caddy/Cloudflare relay as 502.

- kernel: add `parse-safe` — like parse but returns nil on malformed input
  (value-returning, so untrusted text can be handled without a host exception).
- kernel: http-listen now synthesises a 500 response on ANY handler exception
  instead of dropping the connection, so the origin stays responsive (no more
  proxy 502 / branded error page) and the error is logged. This is also the
  only place a native exception can be trapped, since SX guard can't.
- blog: host/blog-render uses (parse-safe sx) — malformed bodies render the
  existing "(unparseable content)" placeholder; the per-block render guard
  already covers unknown components (~kg-*), so /mddddd/ recovers too.

Verified live: /try-thus/ and /mddddd/ now 200 with placeholders; working
posts, home, and login unaffected. 193/193 conformance.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -817,14 +817,28 @@ let setup_evaluator_bridge env =
                (* Run the handler through the IO-aware CEK runner (not bare
                   sx_call) so request handlers can perform per-request IO —
                   durable store reads/writes resolve via cek_run_with_io's
-                  suspension loop instead of returning an unresolved suspension. *)
+                  suspension loop instead of returning an unresolved suspension.
+                  On ANY handler exception, synthesise a 500 response rather than
+                  letting it escape: an escaped exception drops the connection
+                  with no bytes written, which a reverse proxy (Caddy/Cloudflare)
+                  surfaces as a 502 error page. A real 500 keeps the origin
+                  responsive and debuggable. Note: a native exception (e.g. the
+                  parser's Parse_error) cannot be caught by an SX (guard ...), so
+                  this boundary is the only place it can be trapped. *)
                (try
                   let st = Sx_ref.continue_with_call handler
                              (List [Dict req]) (Env (Sx_types.make_env ()))
                              (List [Dict req]) (List []) in
-                  cek_run_with_io st
-                with e -> Mutex.unlock mtx; raise e) in
-             Mutex.unlock mtx;
+                  let r = cek_run_with_io st in
+                  Mutex.unlock mtx; r
+                with e ->
+                  Mutex.unlock mtx;
+                  Printf.eprintf "[http-listen] handler error: %s\n%!"
+                    (Printexc.to_string e);
+                  let d = Sx_types.make_dict () in
+                  Hashtbl.replace d "status" (Integer 500);
+                  Hashtbl.replace d "body" (String "Internal Server Error");
+                  Dict d) in
              let getk k = match resp with
                | Dict h -> Hashtbl.find_opt h k | _ -> None in
              let status = match getk "status" with
@@ -1250,6 +1264,20 @@ let setup_type_constructors env =
       (* Already a value — return as-is *)
       v
     | _ -> raise (Eval_error "parse: expected string"));
+  (* Like parse, but returns nil instead of raising on malformed input. The
+     parser raises a native Parse_error that an SX-level (guard ...) cannot catch
+     (guard only traps SX conditions, not host exceptions), so code that handles
+     untrusted text — e.g. a stored post body — needs a value-returning parse to
+     degrade gracefully rather than crash the request. *)
+  bind "parse-safe" (fun args ->
+    match args with
+    | [String s] | [SxExpr s] ->
+      (try
+         let exprs = Sx_parser.parse_all s in
+         (match exprs with [e] -> e | _ -> List exprs)
+       with _ -> Nil)
+    | [v] -> v
+    | _ -> Nil);
   (* Native bytecode compiler — bootstrapped from lib/compiler.sx *)
   bind "compile" (fun args ->
     match args with [expr] -> Sx_compiler.compile expr | _ -> Nil);