Fix: Correct conditional lock release w/out acquire (#681)

`index_gt::update` uses conditional `lock(node_try_conditional_lock_)` instead
of the hard `lock(node_lock_)` that `index_gt::add` use. When `candidate_slot == updated_slot`,
the function deliberately skips `atomic_set`, but it still returns a `node_conditional_lock_t`
whose destructor unconditionally calls `atomic_reset`.

Closes #680

Co-authored-by: YoungHwi <[email protected]>
Co-authored-by: YoungHwi <[email protected]>
Co-authored-by: Yoonseok Kim <[email protected]>

Author: yoonseok-kim

include/usearch/index.hpp

@@ -3836,7 +3836,11 @@ class index_gt {
 
     inline node_conditional_lock_t node_try_conditional_lock_(std::size_t slot, bool condition,
                                                               bool& failed_to_acquire) const noexcept {
-        failed_to_acquire = condition ? nodes_mutexes_.atomic_set(slot) : false;
+        if (!condition) {
+            failed_to_acquire = false;
+            return {nodes_mutexes_, std::numeric_limits<std::size_t>::max()};
+        }
+        failed_to_acquire = nodes_mutexes_.atomic_set(slot);
         return {nodes_mutexes_, failed_to_acquire ? std::numeric_limits<std::size_t>::max() : slot};
     }