Split ObservableCacheEx.cs into per-family partial classes

[dwcullop]

Splits ObservableCacheEx.cs (6,833 lines, 100 distinct operator names spanning 290 method bodies) into 24 partial-class files grouped by operator family. The two pre-existing partials (ObservableCacheEx.SortAndBind.cs, ObservableCacheEx.VirtualiseAndPage.cs) are untouched.

This is a pure file reorganisation. No code, no XML documentation, no comments, no preprocessor directives, and no constants were added, removed, or altered. The byte content of every method body is preserved. All 2,218 unit tests pass.

New files

Numbers in parentheses indicate the count of overloads.

File	Members
ObservableCacheEx.Adapt.cs	Adapt (2)
ObservableCacheEx.AutoRefresh.cs	AutoRefresh (2), AutoRefreshOnObservable (2)
ObservableCacheEx.Batch.cs	Batch, BatchIf (5)
ObservableCacheEx.Bind.cs	Bind (12), TreatMovesAsRemoveAdd
ObservableCacheEx.ChangeStream.cs	BufferInitial, DeferUntilLoaded (2), SkipInitial, StartWithEmpty (7), StartWithItem (2)
ObservableCacheEx.Combinators.cs	And (5), Except (5), Or (5), Xor (5), Combine (5, private helper)
ObservableCacheEx.Conversions.cs	AsObservableCache (2), Cast, ChangeKey (2), Convert, Flatten, FlattenBufferResult, OfType, RefCount
ObservableCacheEx.Edit.cs	AddOrUpdate (5), Clear (3), Clone, EditDiff (4), InvokeEvaluate, Refresh (3), Remove (6), RemoveKey (2), RemoveKeys, UpdateIndex
ObservableCacheEx.Expiration.cs	ExpireAfter (5), LimitSizeTo (2)
ObservableCacheEx.Filter.cs	EnsureUniqueKeys, Filter (4), FilterImmutable, FilterOnObservable (2), IgnoreSameReferenceUpdate, IgnoreUpdateWhen, IncludeUpdateWhen, NotEmpty, SuppressRefresh, WhereReasonsAre, WhereReasonsAreNot, ForForced (2, private helper)
ObservableCacheEx.Group.cs	Group (5), GroupOnObservable (2), GroupOnProperty, GroupOnPropertyWithImmutableState, GroupWithImmutableState, AdaptSelector (private helper)
ObservableCacheEx.Joins.cs	FullJoin (2), FullJoinMany (2), InnerJoin (2), InnerJoinMany (2), LeftJoin (2), LeftJoinMany (2), RightJoin (2), RightJoinMany (2)
ObservableCacheEx.Lifecycle.cs	AsyncDisposeMany, DisposeMany, FinallySafe, ForEachChange, MonitorStatus, SubscribeMany (2)
ObservableCacheEx.Merge.cs	MergeChangeSets (16), MergeMany (2), MergeManyItems (2), Switch (2)
ObservableCacheEx.MergeManyChangeSets.cs	MergeManyChangeSets (14), DefaultResortOnSourceRefresh const
ObservableCacheEx.Notifications.cs	OnItemAdded (2), OnItemRefreshed (2), OnItemRemoved (2), OnItemUpdated (2), OnChangeAction (2, private helper)
ObservableCacheEx.Populate.cs	PopulateFrom (2), PopulateInto (3)
ObservableCacheEx.PropertyChanged.cs	Watch, WatchValue (2), WhenAnyPropertyChanged, WhenPropertyChanged, WhenValueChanged
ObservableCacheEx.Query.cs	DistinctValues, QueryWhenChanged (3), ToCollection, ToSortedCollection (2), TrueForAll (2), TrueForAny (2), TrueFor (private helper)
ObservableCacheEx.Sort.cs	Sort (4), SortBy, DefaultSortResetThreshold const
ObservableCacheEx.ToObservableChangeSet.cs	ToObservableChangeSet (2), ToObservableOptional (2)
ObservableCacheEx.Transform.cs	Transform (9), TransformAsync (6), TransformImmutable, TransformOnObservable (2), TransformToTree, TransformWithInlineUpdate (4)
ObservableCacheEx.TransformMany.cs	TransformMany (4), TransformManyAsync (6), TransformManySafeAsync (6), CreateChangeSetTransformer (3, private helper)
ObservableCacheEx.TransformSafe.cs	TransformSafe (6), TransformSafeAsync (6)

Deleted

src/DynamicData/Cache/ObservableCacheEx.cs

Verification

The split was generated programmatically with byte-level per-method equality checks against the original. After the move, every public method (and all 14 private static helpers used internally by the public surface) was confirmed to be present in exactly one file in the same order it appeared in the original file. The full test suite (2,218 tests on net9.0) passes.