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>

main.cpp

@@ -2885,17 +2885,16 @@ void set_attribute_value(const char *arg) {
 			exit(EXIT_JSON);
 		}
 
-		for (size_t i = 0; i < o->keys().size(); i++) {
-			json_object_ptr k = o->keys()[i];
-			json_object_ptr v = o->values()[i];
-
-			if (k->type != JSON_STRING) {
+		size_t i = 0;
+		for (const auto &e : o->entries()) {
+			if (e.key->type != JSON_STRING) {
 				fprintf(stderr, "%s: --set-attribute %s: key %zu not a string\n", *av, arg, i);
 				exit(EXIT_JSON);
 			}
 
-			serial_val val = stringify_value(v, "json", 1, o);
-			set_attributes.emplace(k->string(), val);
+			serial_val val = stringify_value(e.value, "json", 1, o);
+			set_attributes.emplace(e.key->string(), val);
+			i++;
 		}
 
 		return;