Perf: Migrate P/Invoke signatures from DllImport to LibraryImport

[MariusStorhaug]

Summary

Adopt source-generated interop (LibraryImport) for libsodium bindings targeting current runtime support.

Why

LibraryImport can reduce runtime marshalling overhead versus reflection-based DllImport, improving hot-path crypto command performance.

Proposed change

• Replace [DllImport] declarations with [LibraryImport] for supported target frameworks.
• Add explicit marshalling annotations where needed (In/Out semantics).
• Validate function signatures and calling convention parity.
• Preserve current API surface exposed to PowerShell.

Acceptance criteria

• Native calls remain functionally equivalent across supported platforms.
• All existing tests pass on Windows/Linux/macOS CI matrices.
• Benchmarks show measurable improvement or no regression.
• No breaking change in public PowerShell cmdlet behavior.