Enable nullable annotations in Ctx.cs

Remove #nullable disable and annotate all nullable types:
- m_reaper is now Reaper? (late-initialized in CreateSocket)
- m_slots is now IMailbox?[]? (late-initialized, elements nullable)
- ChooseIOThread returns IOThread? (already matched by ZObject wrapper)
- Use null-forgiving operator where lifecycle invariants guarantee non-null
- Remove redundant Debug.Assert(addr != null) on non-nullable parameter

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>