[FreeApophis]

Adds Documentation/src/async.md to the mdBook and links it from SUMMARY.md (after the IEnumerable Extensions chapter), from the Sequence constructors chapter and from the IEnumerable Extensions landing page.

The chapter covers:

• where the async APIs live: in Funcky on .NET 10 and newer, in Funcky.Async on older frameworks, and that the net10.0 asset of Funcky.Async only forwards to Funcky, so the reference can be dropped;
• the mental model: Funcky follows the System.Linq.AsyncEnumerable vocabulary, one name per operator with an (item, cancellationToken) delegate, and how to discard the token with (item, _) => …;
• the lazy operators (AdjacentGroupBy, Chunk, ExclusiveScan, InclusiveScan, Inspect, WhereSelect) and the terminal aggregations (AverageOrNoneAsync, FirstOrNoneAsync, LastOrNoneAsync, SingleOrNoneAsync, MaxOrNoneAsync, MinOrNoneAsync, NoneAsync, PartitionAsync) with their delegate shapes;
• the migration from the obsolete …Await / …AwaitWithCancellation / …AwaitAsync / …AwaitWithCancellationAsync overloads (obsolete since 3.8.0, removed in 4.0.0, Remove the obsolete Await/AwaitWithCancellation async operators #981): apply the Funcky.Analyzers 2.1.0 code fix for CS0618 solution-wide, what the fix does with method groups and delegate values, and which calls it leaves alone;
• AsyncSequence, RetryAsync / RetryAwaitAsync, and awaiting Option<Task<T>> / Option<ValueTask<T>> including ConfigureAwait.

Every name and overload was checked against the sources in Funcky/Extensions/AsyncEnumerableExtensions, Funcky/AsyncSequence, Funcky/Functional and Funcky/Monads/Option.

Closes #1018

🤖 Generated with Claude Code

[Copilot]

🟡 Changes recommended

Unresolved documentation errors misstate retargeting compatibility, async-void exception behavior, and supported APIs, and include a broken Memoize link.

5 open issues

• Clarify source compatibility changes from type forwarding · New
• Correct async void exception handling behavior · New
• Use MaterializeAsync in terminal operator examples · New
• Clarify synchronous delegate overload support · New
• Fix broken memoize documentation link · New

What changed in this PR

Adds an async API chapter to the mdBook, addressing #1018 with guidance on package selection, async operators, and migration from obsolete overloads.

Changes:

• Documents framework-specific packaging, callback conventions, and solution-wide migration.
• Covers AsyncSequence, asynchronous retry helpers, and awaiting Option values.
• Adds navigation and cross-links from existing enumerable and sequence documentation.

File	Description
Documentation/​src/​SUMMARY.md	Adds the async chapter to book navigation.
Documentation/​src/​sequence/​sequence.md	Expands async constructor guidance and links to the chapter.
Documentation/​src/​enumerable-extensions/​enumerable-extensions.md	Links async counterpart guidance to the new chapter.
Documentation/​src/​async.md	Introduces async API documentation and migration examples.

_{Review effort: Balanced}