- Add mods_state_changed and mods_removed events, gated to Nexus mods like the
  other per-mod events, carrying a ModChangeReason (user_manual, variant_replace,
  version_update, profile_replace, collection_update, collection_uninstall,
  stop_managing_game, health_check) threaded through IRemoveModOptions and
  IEnableOptions from the remove/enable call sites.
- Add install_kind (fresh / version_update / reinstall / variant / profile_replace)
  to the mods_installation_* events, classified in InstallManager and carried to
  the terminal events via InstallContext.
- Report mods_state_changed.duration_ms as time spent in the prior state in both
  directions; add a durable disabledTime to profile mod-state alongside enabledTime.
- Base mods_download_completed duration_ms on the durable startTime.
- Move ReplaceChoice/IReplaceChoice to their own types file.
- Add test harness (makeModChangeHarness/modChangeTest, makeProfile) and coverage
  for classifyInstallKind and the mod-change analytics.

closes LAZ-673