Restore native packaging project and fix CI solution path

[KSemenenko]

Summary

• restore the MLXSharp.Native packaging project and wire it back into MLXSharp so runtimes ship from a dedicated project
• update the .slnx solution and CI workflow to use the new solution format while keeping native assets copied into MLXSharp.Native
• refresh documentation to point to the reinstated native runtime paths

Testing

• not run (dotnet CLI unavailable in container)

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https://chatgpt.com/codex/tasks/task_e_68e229d761248326b7098a2b8b83fdc3

[Copilot]

Pull Request Overview

Restores a dedicated native packaging project (MLXSharp.Native), updates CI to use a new .slnx solution format, and refreshes documentation to reflect the reinstated native runtime layout. Key structural change is the replacement of the traditional .sln with a custom XML .slnx file referenced throughout CI.

• Reintroduces native packaging project and updates docs to describe stub expansion behavior.
• Replaces MLXSharp.sln with a new MLXSharp.slnx and updates CI to build/test against it.
• Enhances CI package verification logic for native macOS library presence.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/MLXSharp.Native/runtimes/osx-arm64/native/README.txt	Updated placement instruction for macOS dylib (potentially incorrect project reference).
src/MLXSharp.Native/README.md	Rewritten explanation of native stub handling and packaging layout.
README.md	Adjusted repository layout and package contents documentation to reflect restored native packaging approach.
MLXSharp.slnx	Introduces a new non-standard XML solution file replacing the prior .sln.
MLXSharp.sln	Removes the conventional Visual Studio solution file.
.github/workflows/ci.yml	Points restore/build/test to .slnx and expands package validation script.

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