Feature skip plugins mid run

[Abu-Huraira21]

Adds --mp_per_plugin, an opt-in flag that runs each plugin in its own subprocess instead of the main process. Default
behavior (no flag) is unchanged.

Problem: a single hung/stuck plugin blocks the entire run with no way to recover short of killing the process.

Fix: with --mp_per_plugin, each plugin runs in a spawned subprocess. If one hangs:

• Ctrl+C once → skips just that plugin, run continues
• Ctrl+C twice within 2s → aborts the whole run
• kill -USR1 / kill -USR2 (Unix) → skips current plugin externally, no abort risk

Files changed

• ileapp.py — --mp_per_plugin flag, subprocess dispatch, skip signal handlers
• scripts/mp_plugin_runner.py (new) — subprocess entrypoint
• scripts/lavafuncs.py — idempotent LAVA init, open/close helpers for subprocess-safe DB access
• scripts/ilapfuncs.py — picklable seeker/file-info snapshot for passing state into subprocesses

Known issue (not fixed by this PR)

Encrypted iTunes backups can fail to open the decrypted Manifest.db with unable to open database file — pre-existing,
unrelated to this change. Root cause: Manifest.db is WAL-mode, and open_sqlite_db_readonly() has no fallback for opening
WAL-mode DBs missing their -wal/-shm sidecar files. Reproducible on current main independent of --mp_per_plugin. Will
follow up with a separate PR (immutable=1 retry fix).

Testing

Verified --mp_per_plugin end-to-end against an iTunes backup extraction, including plugin skip via Ctrl+C and SIGUSR1.

Notes:

• This PR was made against this request [Feature Request] Runtime key command to skip an artifact #1227