Fix series merging leaking across libraries and under-counting merged children - #17693

Regression from #17417

Series.PresentationUniqueKey no longer carries the owning library, but the queries that resolve a merged series' children are not library-scoped: LibraryManager.AddUserToQuery deliberately skips TopParentIds when SeriesPresentationUniqueKey or AncestorWithPresentationUniqueKey is set. I missed this when I changed the behavior.

Separately, played/total counts for a merged series are taken from the grouping survivor's own subtree, meaning they were wrong too.

Changes

• Restore the owning-library suffix on Series.PresentationUniqueKey, now sorted with StringComparer.Ordinal so sibling folder-items cannot diverge on enumeration order
• Expand played/total counts over the whole presentation-key group
• Count distinct child keys in GetChildCountBatch for merged folders
• Add DescendantQueryHelper.GetAllDescendantIdsBatch so a group's descendants resolve in one traversal
• Rename RecomputeSeriesPresentationKey to a new date prefix and correct it in place to fix existing data
• Add tests for this use case

Code assistance
Claude Opus 5 for testing

Issues
Fixes #17692