v0.9.0

Multi-target net8.0/net9.0 and release v0.9.0

Myriad, Myriad.Core, and Myriad.Plugins now multi-target net8.0 and net9.0
(previously net9.0-only), so consumers on the .NET 8 SDK can build against
them again. net8.0 is LTS through November 2026; a newer SDK can consume an
older-targeted library but not the reverse. Fixes #296.

Myriad.Sdk's hardcoded generator-exe path is updated from tools/net9.0/any/
to tools/net8.0/any/, since that's the slice that must resolve for a
net8.0-only consumer; verified end to end by packing locally, installing the
tool from the local nupkg, and running it.

paket.dependencies packages are now pinned to exact versions instead of
floating, so widening the framework restriction doesn't silently re-resolve
to newer majors (an early attempt let Fantomas.Core/FSharp.Compiler.Service
float to versions with no net8.0 support, and moved the FSharp.Compiler.Service
pin that experiments/ relies on).

VersionPrefix bumped to 0.9.0; CHANGELOG's Unreleased entries moved under it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>