Decide target framework: .NET 10 vs net8 LTS vs multi-target #2

Tracks the "Target framework" open item in PLAN.md §5.

PowerShell#267 bumped the projects to .NET 10. A binary module loads in-process, so its TFM must be compatible with the PowerShell host runtime:

• PowerShell 7.4 → .NET 8
• .NET 10 → requires PowerShell 7.6+

A net10.0-only build narrows the install base to PS 7.6+. Options:

• Keep net10.0 — newest, simplest; PS 7.6+ only.
• Multi-target net8.0;net10.0 — widest reach, larger build/test matrix.
• Pin net8.0 (LTS) — broadest install base, gives up newest runtime.

Decision needed

This gates the rebrand (PowerShellVersion in .psd1) and the PSGallery publish, and feeds the CI matrix.