Define Generic Host, SIGQUIT, and Windows shutdown integration

[autocarl]

Context

Issue #79 suggested using ConsoleLifetime when an IHost is supplied. PR #80 intentionally leaves external IServiceProvider, IHost, and IReplHost overloads caller-owned to avoid duplicate process handlers. It directly covers console cancel keys and Unix SIGTERM for internally configured process-owning profiles, but does not directly register POSIX SIGQUIT or translate Windows service/control shutdown.

Questions to resolve

• Should IHost overloads integrate with IHostApplicationLifetime.ApplicationStopping, ConsoleLifetime, or both?
• Should a future process-owning profile handle SIGQUIT directly, and what exit/second-signal policy should it use?
• What are the correct Windows mappings for Ctrl+C, Ctrl+Break, console close, service stop, taskkill, and job-object termination?
• How can Repl avoid competing with a host that already installed lifetime handlers?

Acceptance criteria

• Hosted and standalone ownership boundaries are explicit and testable.
• No path installs duplicate competing handlers by default.
• SIGQUIT support is either implemented with a documented exit contract or explicitly rejected with rationale.
• Windows-specific lifecycle guidance documents what is catchable, what must come from the host, and what cannot unwind cooperatively.
• Integration tests cover supported Unix and Windows lifecycle events where CI can generate them reliably.