Fix: precise ground/ceiling snap eliminates camera jitter

- Ground snap: scan for first solid below actor, place exactly on surface
- Ceiling snap: scan for first solid above actor, place exactly beneath
- Eliminates 1px gap that caused slow sink cycle → camera wobble fixed
- Also fixes step-up early return preventing fall-through

engine/src/actors.rs

@@ -88,17 +88,29 @@ impl Actor {
 
         if self.collides_at(grid, self.x, new_y) {
             if self.vy > 0.0 {
-                let mut sy = new_y;
-                while sy <= new_y + self.h as f32 && self.collides_at(grid, self.x, sy) {
-                    sy -= 1.0;
+                let mut gy = (self.y + self.h as f32) as u32;
+                let max_y = (new_y + self.h as f32 + 2.0) as u32;
+                while gy <= max_y.min(grid.height) {
+                    if grid.get(self.x.floor() as u32, gy) != 0 {
+                        let props = &crate::material::PROPS[grid.get(self.x.floor() as u32, gy) as usize];
+                        if props.is_solid && !props.is_powder {
+                            self.y = gy as f32 - self.h as f32;
+                            break;
+                        }
+                    }
+                    gy += 1;
                 }
-                self.y = sy;
             } else {
-                let mut sy = new_y;
-                while sy >= new_y - self.h as f32 && self.collides_at(grid, self.x, sy) {
-                    sy += 1.0;
+                let mut gy = (new_y - 2.0).max(0.0) as u32;
+                while gy > 0 {
+                    let mat = grid.get(self.x.floor() as u32, gy);
+                    let props = &crate::material::PROPS[mat as usize];
+                    if props.is_solid && !props.is_powder {
+                        self.y = (gy + 1) as f32;
+                        break;
+                    }
+                    gy -= 1;
                 }
-                self.y = sy;
             }
             self.vy = 0.0;
         } else {