Attack Bug: Crashing aircraft are still attack targets until next slow update

Attack orders on crashing aircraft are not skipped, instead the attacker waits (and doesn't shoot) until the aircraft is dead or the next slow update, causing unnecessary opportunity shots.

Start by tracing attack-order target validation for aircraft and compare the normal update path with the slow-update path described in the issue. Done means crashing aircraft are skipped immediately, so attackers do not wait without shooting until the aircraft dies or the next slow update.

Written by the indexing model from the issue text.