Fix #7848 - Prevent aerodyne aircraft from flying backwards in atmosphere

[HammerGS]

Summary

Fixes #7848

Aerodyne aircraft (fighters, aerospace fighters, aerodyne DropShips) could move backwards in atmosphere using Shift+S keybind, allowing them to fly in reverse while retaining full combat functionality.

Root Cause

The airborne movement legality section in MoveStep.java had no restriction preventing BACKWARDS movement for aerodyne aircraft in atmosphere. While non-Aero airborne units (like LandAirMek in flight mode) were already restricted to only turns and conversion, true Aero units could use any movement type including backwards.

Fix

Added a check in the atmosphere movement rules section (after the turning rules) to prevent backwards movement for aerodyne aircraft:

  ```java
  // aerodynes cannot fly backwards in atmosphere
  if (useAeroAtmosphere(game, entity) &&
        ((type == MoveStepType.BACKWARDS) ||
              (type == MoveStepType.LATERAL_LEFT_BACKWARDS) ||
              (type == MoveStepType.LATERAL_RIGHT_BACKWARDS))) {
      return;
  }

Scope

Unit Type	Affected?	Reason
Conventional Fighter	Yes	Aerodyne, requires forward flight
Aerospace Fighter	Yes	Aerodyne, requires forward flight
Aerodyne DropShip	Yes	Aerodyne, requires forward flight
Spheroid DropShip	No	Hovers like VTOL, different movement
Landed aircraft	No	At altitude 0, different code path
Space movement	No	Different movement rules

[Copilot]

Pull request overview

This PR fixes issue #7848 by preventing aerodyne aircraft from moving backwards while flying in atmosphere. The fix adds a validation check that blocks backwards movement types (BACKWARDS, LATERAL_LEFT_BACKWARDS, LATERAL_RIGHT_BACKWARDS) for aerodyne aircraft operating in atmospheric conditions.

Key Changes:

• Added backwards movement restriction for aerodyne aircraft in atmosphere within the compileIllegal method
• The restriction only applies to aerodynes (not spheroids) when airborne in non-vacuum atmosphere
• Space movement and landed aircraft are unaffected

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