Feature: Side thrusters for drifting / lateral movement

[leighmcculloch]

Description

Add side thrusters separate from the turning/rotation controls, enabling lateral movement and drifting.

Motivation

• The music makes you want to drift — it just fits the vibe.
• Inspired by the Viper from Battlestar Galactica, which has many small thrusters around the nose and body for rotation and movement in multiple directions.
• Even though drifting doesn't make perfect physical sense in space, it feels right for the gameplay experience.

Suggested behavior

• New keybindings (e.g. Q/E or A/D) for lateral thrust, independent of ship rotation.
• Enables strafing and drifting maneuvers — the ship can slide sideways while facing a different direction.