basic missile scene

Minimum features:
- has a collider
- explodes on contact
- explode method (just a blueprint, no behavior)
- movement physics (gravity)
- wind affects it (code some basic wind, horizontal +/- 1 in strength
- for the sprite, something basic like a red triangle

The issue names no files, tests, or entry points, so first inspect the existing Godot scene and projectile patterns in Alpacos. Done means a basic missile scene has a collider, gravity and horizontal wind movement, contact-triggered explosion behavior, an empty explode method blueprint, and a red-triangle sprite.

Written by the indexing model from the issue text.