Store hash key/value pairs in one ordered vector

Replace the parallel std::vector<json_object_ptr> keys / values on
json_hash with a single std::vector<json_entry>, where json_entry is
a small {key, value} aggregate. This still preserves insertion order
(the property the parallel vectors were providing) but removes the
"keep two vectors in lockstep" pattern, and call sites can now use
range-for with structured bindings:

    for (auto &[k, v] : o->entries()) { ... }

Side effects:

* sizeof(json_hash) drops from 72 to 48 bytes (one fewer vector
  header), matching json_array.
* The keys() and values() accessors on json_object are replaced by a
  single entries() accessor returning std::vector<json_entry>&.
* All call sites were swept from the old paired-index pattern
  (`o->keys()[i]` / `o->values()[i]`) to entry-based access. Where the
  original pattern relied on `nprop = 0` to short-circuit iteration on
  a null or non-hash `properties`, the rewrite now guards the loop
  explicitly with `if (o->type == JSON_HASH)` so that calling
  entries() doesn't trip the asserting downcast.

Co-authored-by: Cursor <cursoragent@cursor.com>

jsontool.cpp

@@ -298,8 +298,7 @@ void join_csv(json_object_ptr j) {
 	}
 
 	if (fields.size() > 0 && joinkey == fields[0]) {
-		properties->keys().reserve(properties->keys().size() + fields.size());
-		properties->values().reserve(properties->values().size() + fields.size());
+		properties->entries().reserve(properties->entries().size() + fields.size());
 
 		for (size_t i = 1; i < fields.size(); i++) {
 			std::string k = header[i];
@@ -333,8 +332,7 @@ void join_csv(json_object_ptr j) {
 					abort();
 				}
 
-				properties->keys().push_back(ko);
-				properties->values().push_back(vo);
+				properties->entries().push_back({ko, vo});
 			}
 		}
 	}