fix: cross-fade tiles to sky tile using alpha instead of color tint

Replace df_fade_color (multiplicative tint that preserves texture hue)
with tile_for_depth which cross-fades between real tile and sky tile:

- z_diff <= 0: real tile at full opacity (no fade)
- z_diff in (0, threshold): real tile fades out via alpha
- z_diff past threshold: sky tile (index 0) fades in
- invisible tiles: solid sky colour

Also fix tile_id_to_tileset_index to include sky (id=0) as index 0,
so sky tiles render correctly as the fade target.

src/world/tiles/tilemap_chunk.rs

@@ -69,7 +69,7 @@ pub const FIXTURE_ROW_OFFSET: u16 = 6;
 
 fn tile_id_to_tileset_index(tile_id: u32) -> Option<u16> {
     if tile_id == 0 {
-        return None;
+        return Some(0);
     }
     if tile_id <= 5 {
         return Some(tile_id as u16);
@@ -82,22 +82,44 @@ fn tile_id_to_tileset_index(tile_id: u32) -> Option<u16> {
     }
 }
 
-fn df_fade_color(z_diff: i32) -> Color {
+fn tile_for_depth(real_index: u16, z_diff: i32) -> TileData {
+    const SKY_R: f32 = 133_f32 / 255_f32;
+    const SKY_G: f32 = 167_f32 / 255_f32;
+    const SKY_B: f32 = 178_f32 / 255_f32;
+    const LAYER_ALPHA: f32 = 70_f32 / 255_f32;
+
     if z_diff <= 0 {
-        return Color::WHITE;
+        return TileData {
+            tileset_index: real_index,
+            color: Color::WHITE,
+            visible: true,
+        };
     }
+
     let n = (z_diff as f32).min(8_f32);
-    let overlay_r = 133_f32 / 255_f32;
-    let overlay_g = 167_f32 / 255_f32;
-    let overlay_b = 178_f32 / 255_f32;
-    let layer_alpha = 41_f32 / 255_f32;
-    let acc_alpha = 1_f32 - (1_f32 - layer_alpha).powf(n);
-    let inv = 1_f32 - acc_alpha;
-    Color::srgb(
-        overlay_r * acc_alpha + inv,
-        overlay_g * acc_alpha + inv,
-        overlay_b * acc_alpha + inv,
-    )
+    let t = 1_f32 - (1_f32 - LAYER_ALPHA).powf(n);
+
+    if t >= 0.98 {
+        return TileData {
+            tileset_index: 0,
+            color: Color::srgb(SKY_R, SKY_G, SKY_B),
+            visible: true,
+        };
+    }
+
+    if t < 0.5 {
+        TileData {
+            tileset_index: real_index,
+            color: Color::srgba(1_f32, 1_f32, 1_f32, 1_f32 - t * 2_f32),
+            visible: true,
+        }
+    } else {
+        TileData {
+            tileset_index: 0,
+            color: Color::srgba(SKY_R, SKY_G, SKY_B, (t - 0.5_f32) * 2_f32),
+            visible: true,
+        }
+    }
 }
 
 fn is_tile_visible_at_z(tile_data: &FloorTileData, z_index: usize) -> bool {
@@ -134,19 +156,19 @@ fn populate_chunk_tiles(
             );
             let tile = tilemap.get_floor(&world_pos);
             match tile {
-                Some(t) if tile_id_to_tileset_index(t.id as u32).is_some() => {
-                    let visible = is_tile_visible_at_z(t, z_index);
-                    let color = if visible {
-                        df_fade_color(z_diff)
+                Some(t) => {
+                    let idx = tile_id_to_tileset_index(t.id as u32);
+                    if let Some(tileset_idx) = idx {
+                        let is_visible = is_tile_visible_at_z(t, z_index);
+                        let tile_data = if is_visible {
+                            tile_for_depth(tileset_idx, z_diff)
                         } else {
-                        Color::BLACK
+                            tile_for_depth(0, 999)
                         };
-                    let tileset_idx = tile_id_to_tileset_index(t.id as u32).unwrap();
-                    tiles.push(Some(TileData {
-                        tileset_index: tileset_idx,
-                        color,
-                        visible,
-                    }));
+                        tiles.push(Some(tile_data));
+                    } else {
+                        tiles.push(None);
+                    }
                 }
                 _ => tiles.push(None),
             }