Split ObservableListEx.cs into per-family partial classes

[dwcullop]

Splits ObservableListEx.cs (2,929 lines, 63 distinct operator names spanning 128 method bodies) into 17 partial-class files grouped by operator family.

This is a pure file reorganisation. The only code change is converting public static class ObservableListEx to public static partial class ObservableListEx. No method bodies, XML documentation, comments, preprocessor directives, or other content was 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
ObservableListEx.Adapt.cs	Adapt
ObservableListEx.AutoRefresh.cs	AutoRefresh (2), AutoRefreshOnObservable
ObservableListEx.Bind.cs	Bind (5)
ObservableListEx.ChangeStream.cs	BufferIf (4), BufferInitial, DeferUntilLoaded (2), NotEmpty, SkipInitial, StartWithEmpty
ObservableListEx.Combinators.cs	And (5), Except (5), Or (5), Xor (5), Combine (5, private helper)
ObservableListEx.Conversions.cs	AddKey, AsObservableList (2), Cast (2), CastToObject, Clone, Convert, FlattenBufferResult, ForEachChange, ForEachItemChange, RefCount, RemoveIndex
ObservableListEx.Expiration.cs	ExpireAfter, LimitSizeTo, Top
ObservableListEx.Filter.cs	DistinctValues, Filter (3), FilterOnObservable, FilterOnProperty, SuppressRefresh, WhereReasonsAre, WhereReasonsAreNot
ObservableListEx.Group.cs	GroupOn, GroupOnProperty, GroupOnPropertyWithImmutableState, GroupWithImmutableState
ObservableListEx.Lifecycle.cs	DisposeMany, PopulateInto, SubscribeMany
ObservableListEx.Merge.cs	MergeChangeSets (10), MergeMany, MergeManyChangeSets (3), Switch (2)
ObservableListEx.Notifications.cs	OnItemAdded, OnItemRefreshed, OnItemRemoved
ObservableListEx.Pagination.cs	Page, Virtualise
ObservableListEx.PropertyChanged.cs	WhenAnyPropertyChanged, WhenPropertyChanged, WhenValueChanged
ObservableListEx.Query.cs	QueryWhenChanged (2), ToCollection, ToObservableChangeSet (8), ToSortedCollection (2)
ObservableListEx.Sort.cs	Reverse, Sort (2)
ObservableListEx.Transform.cs	Transform (4), TransformAsync (4), TransformMany (4)

Deleted

src/DynamicData/List/ObservableListEx.cs

Verification

The split was generated programmatically with byte-level per-method equality checks against the original. After the move, every public method (and the 5 private static Combine overloads used internally by the combinator operators) 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.