refactor!: remove v3 binary-compatibility shims for TUnit

[thomhurst]

Summary

TUnit no longer depends on EnumerableAsyncProcessor, so the v3 binary-compatibility shims kept for TUnit.Engine can go before v4 ships.

• Remove the redundant ProcessInParallel(int maxConcurrency) shims from ActionAsyncProcessorBuilder, ActionAsyncProcessorBuilder<TOutput>, ItemActionAsyncProcessorBuilder<TInput>, and ItemActionAsyncProcessorBuilder<TInput, TOutput>. Callers passing an int still compile via the implicit conversion to the ProcessInParallel(int?, bool) overload.
• Remove the obsolete no-selector AsyncEnumerableExtensions.ProcessInParallel<T>(IAsyncEnumerable<T>, CancellationToken) overload, which only collected the stream into a list.
• Delete V3BinaryCompatibilityTests and the five matching PublicAPI.Shipped.txt entries (v4 is untagged, so this surface was never actually shipped).
• Drop the TUnit coupling guidance from CLAUDE.md and update the README v3→v4 migration note.

Test plan

• dotnet build — clean, no new warnings
• dotnet test — 1716 tests pass across net8.0/net9.0/net10.0; test discovery succeeds, confirming TUnit.Engine no longer binds the removed members

[greptile-apps]

Greptile Summary

This PR removes pre-v4 binary-compatibility shims from the public API. The main changes are:

• Removes four builder ProcessInParallel(int) shims.
• Removes the no-selector async-enumerable overload.
• Deletes the compatibility test and matching API entries.
• Updates TUnit guidance and v4 migration documentation.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• Integer builder calls retain equivalent source behavior through the remaining int? overload.
• The intentional source-breaking extension removal is documented in the migration guide.
• Successful test discovery addresses the former TUnit runtime-binding path.

Important Files Changed

Filename	Overview
EnumerableAsyncProcessor/Builders/ActionAsyncProcessorBuilder.cs	Removes the integer compatibility shim while source calls continue to use the nullable-concurrency overload.
EnumerableAsyncProcessor/Builders/ActionAsyncProcessorBuilder_1.cs	Removes the equivalent compatibility shim from the result builder.
EnumerableAsyncProcessor/Builders/ItemActionAsyncProcessorBuilder_1.cs	Removes the equivalent compatibility shim from the item-action builder.
EnumerableAsyncProcessor/Builders/ItemActionAsyncProcessorBuilder_2.cs	Removes the equivalent compatibility shim from the item-result builder.
EnumerableAsyncProcessor/Extensions/AsyncEnumerableExtensions.cs	Removes the obsolete no-selector overload that only collected the stream.
EnumerableAsyncProcessor/PublicAPI.Shipped.txt	Removes the five matching entries from the public API baseline.
EnumerableAsyncProcessor.UnitTests/V3BinaryCompatibilityTests.cs	Deletes tests for compatibility contracts no longer retained in v4.
README.md	Updates the migration guide to document removal of all no-selector overloads.

_{Reviews (1): Last reviewed commit: "refactor!: remove v3 binary-compatibilit..." | Re-trigger Greptile}