fix(runtime): validate engine installation completion and preserve live install state (fixes #112)

[BerryUIKI]

Closes #112.

Changes

• Track active installation workers (_active_installs) in \IsolatedEngineInstaller\ so live status polling reads (
  ead_manifest) remain side-effect-free and do not erroneously mark in-progress installations as \INTERRUPTED.
• Enforce per-engine exclusive async lease locks on \install_engine\ via \ ask_registry.get_install_lock, rejecting overlapping concurrent operations.
• Recover and clean up corrupt or empty engine directories lacking valid entrypoints (\main.py, \launch.py/\webui.py) or .git\ before attempting repository clone.
• Added strict post-installation validation (\�alidate_installation) verifying the hermetic Python binary, expected entrypoints, and
  equirements.txt\ before marking \COMPLETED.
• Added regression tests in \ est_m2_engines.py\ verifying live polling stability, empty directory/missing entrypoint rejection, and per-engine lease locking.