fix: dig_rng sequential multiply-add mixing

Switch from add-then-XOR pattern to proper sequential multiply-add
avalanche. Each coordinate feeds into the next multiply, preventing
correlation between adjacent tiles when rare drops are introduced.

src/entities/item/drop_table.rs

@@ -54,12 +54,16 @@ impl DropTable {
 
 pub fn dig_rng(pos: IVec3) -> u32 {
     let mut h = (SEED as u64)
-        .wrapping_add(pos.x as u64)
         .wrapping_mul(0x9e3779b97f4a7c15)
-        ^ (pos.y as u64).wrapping_mul(0x6c62272e07bb0142)
-        ^ (pos.z as u64).wrapping_mul(0x94d049bb133111eb);
-    let h32 = (h ^ (h >> 32)) as u32;
-    h32 ^ (h32 >> 16)
+        .wrapping_add(pos.x as u64)
+        .wrapping_mul(0x6c62272e07bb0142)
+        .wrapping_add(pos.y as u64)
+        .wrapping_mul(0x94d049bb133111eb)
+        .wrapping_add(pos.z as u64);
+    h ^= h >> 32;
+    h = h.wrapping_mul(0xbf58476d1ce4e5b9);
+    h ^= h >> 32;
+    h as u32
 }
 
 #[derive(Deserialize)]