feat: square cells (16x16), slope stepping collision

Square cells:
- CHAR_W and CHAR_H both 16 (was 8x16, non-square)
- Window size updated to 160*16 x 50*16 = 2560x800
- Applies to both ascii and graphics renderers

Slope stepping collision:
- Before resolving X, check if new position overlaps solid
- If overlap, try stepping up 1 cell — if clear, snap up (walk up slope)
- If can't step up, resolve X normally (wall block)
- aabb_overlaps_solid() helper for fast overlap check
- Player can now walk up 1-cell-high steps and slopes

109 tests, 0 failures

src/render/vulkan.rs

@@ -8,7 +8,7 @@ use crate::world::cell::MaterialId;
 use crate::world::grid::Grid;
 use crate::world::material::MaterialRegistry;
 
-const CHAR_W: u32 = 8;
+const CHAR_W: u32 = 16;
 const CHAR_H: u32 = 16;
 const ATLAS_COLS: usize = 16;
 const ATLAS_ROWS: usize = 16;