v0.4: Acid system rework + Iron material + heat conductivity

- Acid now uses acid_resist + health-based damage (not instant delete)
- Acid dissolve speeds: Wood ~2s, Dirt ~2.8s, Stone ~4.3s, Iron immune
- New material: Iron (ID 16) - silver, static, acid-proof, heat-conductive
- Iron texture: bright highlights for metallic shine
- Iron reflects light (light_block: 150)
- Iron heat_conduct: 200 - conducts heat quickly through entire block
- Improved heat transfer formula (divisor 255→200)
- Updated AGENTS.md status
- No player code (rolled back for fresh start)

engine/src/render_buffer.rs

@@ -229,6 +229,17 @@ fn texture_offset(x: u32, y: u32, mat: u8) -> i32 {
             let n = hash(x, y.wrapping_mul(y));
             ((n as i32 - 128) * 5) >> 7
         }
+        16 => {
+            let n = hash(x ^ (y >> 2), y ^ (x >> 2));
+            let highlight = ((x.wrapping_mul(13) ^ y.wrapping_mul(7)) & 31) as i32;
+            if highlight < 3 {
+                25 + (highlight * 6)
+            } else if highlight < 6 {
+                14
+            } else {
+                ((n as i32 - 128) * 10) >> 7
+            }
+        }
         _ => 0,
     }
 }