Length-prefixed binary framing for OCaml↔Python pipe

Replace newline-delimited text protocol with length-prefixed blobs
for all response data (send_ok_string, send_ok_raw). The OCaml side
sends (ok-len N)\n followed by exactly N raw bytes + \n. Python reads
the length, then readexactly(N).

This eliminates all pipe desync issues:
- No escaping needed for any content (HTML, SX with newlines, quotes)
- No size limits (1MB+ responses work cleanly)
- No multi-line response splitting
- No double-escaping bugs

The old (ok "...") and (ok-raw ...) formats are still parsed as
fallbacks for backward compatibility.

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

hosts/ocaml/bin/sx_server.ml

@@ -68,24 +68,24 @@ let send line =
 
 let send_ok () = send "(ok)"
 let send_ok_value v = send (Printf.sprintf "(ok %s)" (serialize_value v))
-let send_ok_string s = send (Printf.sprintf "(ok \"%s\")" (escape_sx_string s))
 let send_error msg = send (Printf.sprintf "(error \"%s\")" (escape_sx_string msg))
 
-(** Send ok-raw, ensuring single-line output.
-    SX wire format from aser may contain newlines inside string literals.
-    We must escape those to prevent pipe desync (Python reads one line
-    at a time), but we can't blindly replace newlines in the raw SX
-    because that would break string content.
+(** Length-prefixed binary send — handles any content without escaping.
+    Sends: (ok-len N)\n followed by exactly N bytes of raw data, then \n.
+    Python reads the length line, then reads exactly N bytes. *)
+let send_ok_blob s =
+  let n = String.length s in
+  Printf.printf "(ok-len %d)\n" n;
+  print_string s;
+  print_char '\n';
+  flush stdout
 
-    Strategy: wrap as a properly escaped string literal.
-    Python side will unescape it. *)
-let send_ok_raw s =
-  (* If the result has no newlines, send as-is for backward compat *)
-  if not (String.contains s '\n') then
-    send (Printf.sprintf "(ok-raw %s)" s)
-  else
-    (* Wrap as escaped string so newlines are preserved *)
-    send (Printf.sprintf "(ok \"%s\")" (escape_sx_string s))
+(** Send a string value — use blob for anything that might contain
+    newlines, quotes, or be large. *)
+let send_ok_string s = send_ok_blob s
+
+(** Send raw SX wire format — may contain newlines in string literals. *)
+let send_ok_raw s = send_ok_blob s
 
 
 (* ====================================================================== *)