[SortedCollections] Fix incorrect offset calculation in BTree.findAnyIndex

[brianchang928]

This PR fixes an issue where findAnyIndex(forKey:) could return an Index with an incorrect global offset when the matching key was found in an internal B-tree node. While the key lookup itself was correct, the reported offset undercounted the number of elements before the match.

The root cause was that the offset calculation only added the number of preceding keys in the node but did not include the sizes of the preceding child subtrees, including the child at keySlot, which in in-order traversal all come before the matching key.

This PR ensures the calculation to also include the sizes of all child subtrees up to and including handle[childAt: keySlot] when the match is found in an internal node. This ensures Index.offset always matches the true in-order position, keeping index-based operations consistent.

Resolves #138, resolves #366

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.

[lorentey]

Thank you!

[lorentey]

@swift-ci test

[lorentey]

@swift-ci test