Add per-level wind atmosphere property

WIND directive in .lvl files sets a constant horizontal force (px/s^2)
that pushes entities, projectiles, and particles. Positive is rightward.

Wind is applied as acceleration in physics_update() (halved on ground),
directly to projectile and particle velocities, and as a gentle position
drift on flyers. Entities with gravity_scale=0 (drones, spacecraft) are
unaffected. Levels default to no wind when the directive is absent.

src/engine/particle.c

@@ -82,6 +82,7 @@ void particle_emit(const ParticleBurst *b) {
 
 void particle_update(float dt) {
     float gravity = physics_get_gravity();
+    float wind    = physics_get_wind();
 
     for (int i = 0; i < MAX_PARTICLES; i++) {
         Particle *p = &s_particles[i];
@@ -96,6 +97,11 @@ void particle_update(float dt) {
         /* Apply gravity */
         p->vel.y += gravity * p->gravity_scale * dt;
 
+        /* Apply wind (reuse gravity_scale as environmental-force scale) */
+        if (wind != 0.0f && p->gravity_scale > 0.0f) {
+            p->vel.x += wind * p->gravity_scale * dt;
+        }
+
         /* Apply drag */
         if (p->drag > 0) {
             float factor = 1.0f - p->drag * dt;