Add PowerShell graph and MNIST notebooks

[eosfor]

Summary

• add a PowerShell Gallery sample for PSQuickGraph and PSGraphView
• add the PowerShell Micrograd notebook and its helper scripts
• add an MNIST notebook that uses PSMnist with the Micrograd model
• load exact tested prerelease module versions without requiring PowerShellGet inside the embedded kernel
• use iterative post-order traversal for backpropagation so MNIST-sized graphs do not exceed the PowerShell call-depth limit

Runtime compatibility

The notebooks are verified with the current Verso PowerShell kernel on .NET 8. Their published module dependencies target .NET 8, and the notebook code uses stable PowerShell language and module-loading APIs rather than Verso host or SDK-specific APIs.

They are therefore expected to continue working when Verso moves to a newer PowerShell SDK and .NET runtime. That future combination should still be revalidated when the kernel upgrade lands; this PR intentionally does not change Verso.Host, the PowerShell SDK, or VS Code behavior.

Validation

• all 65 Verso.PowerShell.Tests tests pass on net8.0
• verso run --fail-fast executes the complete Gallery notebook: 11/11 cells
• verso run --fail-fast executes the complete Micrograd notebook: 26/26 cells
• verso run --fail-fast executes the complete MNIST notebook: 15/15 cells, including training and evaluation
• all three module setup cells execute through Verso.PowerShell.Kernel.PowerShellKernel, using the embedded InitialSessionState.CreateDefault2() runspace
• all added notebook files parse as valid JSON, and all PowerShell cells pass parser validation
• setup cells contain no calls to Install-Module or Get-InstalledModule; missing dependencies produce terminal installation instructions instead

All commits include DCO sign-off.

[TorreyBetts]

Nice PowerShell sample, thanks for putting it together. I've been told that legally the Value/Neuron/Layer/MLP port follows Karpathy's micrograd closely enough that it should carry attribution, so could you add a short header comment to value.ps1 and neuronHelper.ps1 crediting micrograd along with its MIT notice?

[eosfor]

Thanks for pointing this out. I've added attribution headers to both value.ps1 and neuronHelper.ps1, crediting Andrej Karpathy's micrograd, preserving the original copyright notice, and linking to its MIT license. Addressed in commit 8633616.