Summary

• Native AOT fails to load global/system git config because sequential-layout class GitBuf marshalling does not write ptr/size back to managed code after git_config_find_*.
• That makes Configuration.BuildSignature return null when user.name / user.email are only in global config (including via include.path).
• Introduce blittable GitBufNative and use ref P/Invoke for path-find APIs and ConvertPath, so config discovery matches CoreCLR/dotnet run behavior.

Root cause (brief)

Under Native AOT, class-based sequential marshalling is effectively [In]-only. git_config_find_global can return success while managed GitBuf.ptr stays zero, so discovery returns no path and only local repo config is loaded.

Test plan

• Build LibGit2Sharp for all TFMs
• AOT publish of a small app using repo.Config.BuildSignature: HasConfig(Global) / HasConfig(System) true, signature non-null when identity is in global config
• CoreCLR / dotnet run path still resolves global config and builds signature
• CI on this PR

Fixes #2187