fix: lerp RGB toward sky colour with full opacity — no alpha fade

- tile_for_depth: alpha always 1.0, lerp RGB from white toward sky RGB
  instead of fading tile out via alpha. Tiles stay opaque.
- Expose LAYER_ALPHA as pub const so it can be tuned at runtime
- LAYER_ALPHA changed from 41/255 to 50/255 for steeper visible fade
- Invisible tiles: explicit solid sky colour, not tile_for_depth(0, 999)

src/world/tiles/tilemap_chunk.rs

@@ -66,6 +66,7 @@ impl TilemapChunkSpawner {
 }
 
 pub const FIXTURE_ROW_OFFSET: u16 = 6;
+pub const LAYER_ALPHA: f32 = 50_f32 / 255_f32;
 
 fn tile_id_to_tileset_index(tile_id: u32) -> Option<u16> {
     if tile_id == 0 {
@@ -86,7 +87,6 @@ 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 TileData {
@@ -107,18 +107,13 @@ fn tile_for_depth(real_index: u16, z_diff: i32) -> TileData {
         };
     }
 
-    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,
-        }
+    let r = (1_f32 - t) + SKY_R * t;
+    let g = (1_f32 - t) + SKY_G * t;
+    let b = (1_f32 - t) + SKY_B * t;
+    TileData {
+        tileset_index: real_index,
+        color: Color::srgb(r, g, b),
+        visible: true,
     }
 }
 
@@ -163,7 +158,15 @@ fn populate_chunk_tiles(
                         let tile_data = if is_visible {
                             tile_for_depth(tileset_idx, z_diff)
                         } else {
-                            tile_for_depth(0, 999)
+                            TileData {
+                                tileset_index: 0,
+                                color: Color::srgb(
+                                    133_f32 / 255_f32,
+                                    167_f32 / 255_f32,
+                                    178_f32 / 255_f32,
+                                ),
+                                visible: true,
+                            }
                         };
                         tiles.push(Some(tile_data));
                     } else {