decide & spike: fold postguard-dotnet into postguard (ship pg-ffi and bindings together)

[rubenhensen]

Decision issue. pg-ffi's build.sh already copies binaries into ../postguard-dotnet; shipping bindings and binaries from one repo removes the version-sync problem at the root (interim mitigation: the pin in encryption4all/postguard-dotnet). Against: different toolchain, NuGet publishing move. Spike and record the decision here.

Part of #247 (workstream F).