refactor!: remove result-discarding InParallelAsync overloads and polish packaging

[thomhurst]

Summary

Pre-4.0 API/packaging polish from the final release review:

• Remove the result-selector InParallelAsync overloads (Func<TSource, Task<TResult>> x2 and Func<TSource, TResult>). They returned a plain Task and silently discarded every result. Results belong to SelectAsync(...).ProcessInParallel(...); existing calls passing result-returning delegates still compile against the surviving Func<TSource, Task> / Action<TSource> overloads with identical behaviour. PublicAPI.Shipped.txt trimmed to match (v4 surface is unreleased) and a migration bullet added to the README.
• XML docs for all remaining ParallelExtensions members and a rewrite of the non-generic IAsyncProcessor docs from legacy /** */ blocks to /// style matching IAsyncProcessor<TOutput>.
• NuGet package icon (new icon.png, packed via PackageIcon).
• Refreshed package description to cover IAsyncEnumerable streaming, timed rate limiting, and the fluent API.

Test plan

• dotnet build clean (0 errors; RS0016/RS0017 public-API analyzers stay as errors)
• dotnet test — 1716/1716 pass on net8.0, net9.0, net10.0
• dotnet pack -c Release succeeds; verified icon.png and README.md land in the nupkg root

[greptile-apps]

Greptile Summary

This PR prepares the public API and package for the v4 release. The main changes are:

• Removes result-discarding InParallelAsync overloads.
• Updates the public API manifest and migration guidance.
• Adds XML documentation for the remaining parallel APIs and IAsyncProcessor.
• Adds a NuGet package icon and refreshes the package description.

Confidence Score: 5/5

This looks safe to merge.

• The migration guidance now describes the source compatibility limits.
• The cancellation documentation matches the intended fault-precedence contract.
• No blocking issue remains within the follow-up scope.

Important Files Changed

Filename	Overview
EnumerableAsyncProcessor/Extensions/ParallelExtensions.cs	Removes the result-selector overloads and documents the remaining parallel-processing API.
README.md	Updates the v4 migration notes for removed overloads and delegate compatibility.
EnumerableAsyncProcessor/PublicAPI.Shipped.txt	Removes the deleted overloads from the tracked public API.
EnumerableAsyncProcessor/EnumerableAsyncProcessor.csproj	Adds package icon metadata, includes the icon in the package, and refreshes the description.
EnumerableAsyncProcessor/Interfaces/IAsyncProcessor.cs	Rewrites the interface documentation using standard XML comments.

_{Reviews (2): Last reviewed commit: "docs: correct InParallelAsync migration ..." | Re-trigger Greptile}

[thomhurst]

Both review comments addressed:

• P1 (source compatibility): README migration bullet rewritten — non-async lambdas returning Task<T> still bind to Func<TSource, Task>, but async value-returning lambdas and explicitly typed result delegates must be rewritten to discard the result.
• P2 (faults vs cancellation): XML doc on the cancellationToken parameters now states that item failures take precedence over cancellation on the returned task.