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)

web/src/main.ts

@@ -6,7 +6,7 @@ import { Camera } from "./camera";
 const GRID_WIDTH = 4096;
 const GRID_HEIGHT = 3072;
 
-const PAINT_MATERIALS = [M.SAND, M.WATER, M.LAVA, M.WOOD, M.FIRE, M.ICE, M.OIL, M.ACID, M.STONE, M.DIRT, M.GLASS];
+const PAINT_MATERIALS = [M.SAND, M.WATER, M.LAVA, M.WOOD, M.FIRE, M.ICE, M.OIL, M.ACID, M.IRON, M.STONE, M.DIRT, M.GLASS];
 
 async function main() {
   const canvas = document.getElementById("canvas") as HTMLCanvasElement;