Phase 1 of the s6-overlay supervision plan. Pure-refactor addition:
introduces the abstract interface (with runtime_checkable Protocol),
detect_service_manager(), validate_profile_name(), and thin
SystemdServiceManager / LaunchdServiceManager / WindowsServiceManager
wrappers around the existing systemd_* / launchd_* / gateway_windows.*
module-level functions. No host call site was modified — host code
continues to use the existing functions directly; the protocol is for
new backend-agnostic code (Phase 4 profile create/delete hooks and the
Phase 4 s6 dispatch path in 'hermes gateway start/stop/restart').
WindowsServiceManager.install() forwards the v3 kwargs (start_now,
start_on_login, elevated_handoff) added in PRs #28169-adjacent so
non-Windows callers — there aren't any today — can opt in.
The s6 backend lands in Phase 3; until then get_service_manager()
raises a clear error if invoked on a host that detects as 's6'.
