When/how do I unlock the light artillery turret?

You can research it after you've researched explosives, plastics and military science!

What are the stats of the light artillery turret?

Minimum range of 32
Maximum automatic range of 168
Manual range of 420
Shooting speed of 0.3/s

What about the light artillery wagon?

It's the light artillery turret gun, but on a train wagon, meaning it has the same stats/range as the standalone turret. The wagon only weighs 2000 units as opposed to the 4000 that the vanilla artillery wagon weighs.

How are the light artillery shells different from normal artillery shells?

They only deal 100 physical damage, as opposed to 500 physical and 500 explosive. Their impact radius is also smaller, at only 2. They are however cheaper to craft, but don't reveal terrain. This is intended to make turrets use, on average, 5 shells to kill a spawner.