loader: emit traffic body as nested JSON, not escaped string

Body field in traffic.ndjson / battle-traffic.ndjson is now a real nested
JSON object instead of an escaped JSON string. Parse-on-write is wrapped in
ParseBodyOrKeep with a string-fallback so a future non-JSON caller doesn't
crash the line write.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSimLoader/CaptureWriter.cs

@@ -91,7 +91,7 @@ internal static class CaptureWriter
             { "direction", direction },
             { "url", url },
             { "encrypted", encrypted },
-            { "body", body },
+            { "body", ParseBodyOrKeep(body) },
         });
     }
 
@@ -102,7 +102,7 @@ internal static class CaptureWriter
             { "ts", DateTime.UtcNow.ToString("o") },
             { "direction", direction },
             { "uri", uri },
-            { "body", body },
+            { "body", ParseBodyOrKeep(body) },
         });
     }
 
@@ -278,6 +278,17 @@ internal static class CaptureWriter
         if (classes.Count > 0) dump["classes"] = classes;
     }
 
+    // Parse body JSON so it serializes nested in the NDJSON line rather than as an
+    // escaped string. Falls back to the original string on parse failure — no current
+    // caller produces non-JSON, but a future caller passing raw text shouldn't crash
+    // the line write.
+    private static object ParseBodyOrKeep(string body)
+    {
+        if (string.IsNullOrEmpty(body)) return body;
+        try { return JsonMapper.ToObject(body); }
+        catch { return body; }
+    }
+
     private static void AppendNdjson(string path, Dictionary<string, object> entry)
     {
         string line = JsonMapper.ToJson(entry);