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>

shared/sx/ocaml_bridge.py

@@ -497,6 +497,13 @@ class OcamlBridge:
         Returns (kind, value) where kind is "ok" or "error".
         """
         line = await self._readline()
+        # Length-prefixed blob
+        if line.startswith("(ok-len "):
+            n = int(line[8:-1])
+            assert self._proc and self._proc.stdout
+            data = await self._proc.stdout.readexactly(n)
+            await self._proc.stdout.readline()  # trailing newline
+            return ("ok", data.decode())
         return _parse_response(line)
 
     async def _read_until_ok(
@@ -547,6 +554,15 @@ class OcamlBridge:
                 pending_batch = []
                 continue
 
+            # Length-prefixed blob: (ok-len N)
+            if line.startswith("(ok-len "):
+                n = int(line[8:-1])
+                assert self._proc and self._proc.stdout
+                data = await self._proc.stdout.readexactly(n)
+                # Read trailing newline
+                await self._proc.stdout.readline()
+                return data.decode()
+
             kind, value = _parse_response(line)
             if kind == "error":
                 raise OcamlBridgeError(value or "Unknown error")