Feat: Introduce selectable installation policies (stable-main & latest-active)

This major release introduces user-selectable installation strategies,
giving users direct control over their environment's stability.

Users can now choose between two powerful modes in their config:

1. 'stable-main' (Default): Creates a fortress-like stable environment.
   Any upgrade or downgrade to an existing package (and its dependencies)
   is automatically isolated into a bubble, leaving the main
   environment untouched. This is ideal for reproducibility and production.

2. 'latest-active': For users who want the latest versions of packages
   they request to be active, while still being protected from
   unintended dependency downgrades, which are bubbled.

This release also hardens the core engine, making omnipkg fully
self-protecting. It now recursively finds its own dependencies to
prevent accidental uninstallation.

Author: 1minds3t