This PR fixes an issue where calling remove(at:) on a SortedSet would correctly remove the element but fail to update the set’s count, leading to inconsistency between the actual contents and the reported size of the set.

The root cause was a missing update to the internal B-Tree node’s subtreeCount when removing an element from a node's child node. This PR ensures the internal node properly decrements its subtreeCount after removal, keeping the tree metadata consistent.

Resolves #441

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.