When removing elements from a B-tree, the root node could become empty while still having children. The previous code incorrectly just called drop() which deallocated all children, causing massive data loss during removal operations.

This fix implements proper root reduction by moving the single remaining child out of the empty root, then safely dropping the now-empty parent node and promoting the extracted child to become the new root.

This fixes test_modifySubscriptRemoval in SortedDictionaryTests, so we re-enable SortedDictionary tests which now all pass.

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.