Remove a dynamically created stackalloc

[adamhathcock]

Hopefully there's no more that will blow out the stack.

Fixes #965

[Copilot]

Pull Request Overview

This PR replaces dynamically-sized stackalloc allocations with ArrayPool<byte> to prevent potential stack overflow issues. The changes address issue #965 by eliminating stack allocations whose size could vary based on input data.

Key Changes:

• Removed unused DDecode and DBits stackalloc declarations in Unpack29 method
• Replaced dynamic stackalloc byte[VMCodeSize] with ArrayPool<byte>.Shared.Rent() in AddVMCode method
• Removed unused length parameter from AddVMCode method

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