Linux packaging: `Stride.FreeImage` and `Stride.Importer.3D` ship no Linux natives — asset compile fails until undocumented `apt` packages are installed

Release Type: Official Release
Version: 4.3.0.2507
Platform(s): Linux (code-only asset compilation)

Describe the bug
Two NuGet packages the asset compiler needs at build time ship Windows natives only:

• Stride.FreeImage: texture compile fails until system libfreeimage3 + libfreeimage-dev
  are installed — with zero diagnostics pointing there.
• Stride.Importer.3D (Assimp): .sdm3d model compile fails with
  Could not load native library libassimp until apt install libassimp-dev — the -dev
  package specifically, because the loader probes the unversioned .so symlink.

To Reproduce
Fresh Linux machine, code-only project with one texture and one model asset, dotnet build.

Expected behavior
Either bundled linux-x64 natives in the packages, or a clear diagnostic + documented Linux
prerequisites.

Additional context
Happy to PR either the packaging (bundle the linux-x64 .sos) or a docs update listing the
Linux prerequisites for code-only asset compilation.

Start by reproducing dotnet build on a fresh Linux machine with a texture and .sdm3d model, then inspect the Stride.FreeImage and Stride.Importer.3D package contents and native loading behavior. Done means Linux code-only asset compilation works with bundled natives, or clearly reports and documents the required libfreeimage and libassimp-dev prerequisites.