Add ArrayPool<T>.RentOwner() extension returning IMemoryOwner<T>

[Copilot]

ArrayPool<T> lacks the IDisposable-based ownership pattern that MemoryPool<T> provides, making it awkward to use in using scopes without manual Return() calls.

Changes

• ArrayPoolExtensions.cs: adds RentOwner(int minimumLength = 1) extension on ArrayPool<T>, returning an IMemoryOwner<T> that returns the buffer to the pool on dispose. Memory.Length is sliced to exactly minimumLength, matching MemoryPool<T> semantics. Disposal is idempotent.
• ArrayPoolExtensionsTests.cs: tests for correct Memory.Length and buffer return on dispose.

Usage

using var owner = ArrayPool<byte>.Shared.RentOwner(81920);
// owner.Memory is exactly 81920 bytes
// buffer is returned to the pool when owner is disposed

[Copilot]

Pull request overview

Adds an ArrayPool<T> extension that enables using-style ownership (IMemoryOwner<T>) for rented arrays, aligning the ergonomics with MemoryPool<T> usage patterns in the codebase.

Changes:

• Introduces ArrayPool<T>.RentOwner(int minimumLength = 1) that returns an IMemoryOwner<T> and returns the rented array on dispose.
• Slices Memory to exactly minimumLength elements (even if a larger array is rented).
• Adds unit tests for Memory.Length and dispose behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
PowerKit/Extensions/ArrayPoolExtensions.cs	Adds RentOwner() and an IMemoryOwner<T> implementation that returns arrays to ArrayPool<T> on disposal.
PowerKit.Tests/ArrayPoolExtensionsTests.cs	Adds tests covering the new RentOwner() API shape and intended dispose semantics.

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