v0.5: Blue sky gradient, acid rework, iron, phase transitions, shadows (experimental)

- Blue sky gradient background (y<600 blue, 600-700 gradient, y>700 dark)
- Acid system: health-based damage via acid_resist, not instant delete
- New material: Iron (ID 16) - acid-proof, heat-conductive, reflective
- Phase transitions: Iron→Lava (150°C), Sand→Glass (200°C), Glass→Lava (160°C)
- Stone no longer melts (lava containment)
- Heat transfer formula improved (divisor 255→200)
- Iron metallic texture with bright highlights + light reflection
- Shadow system attempted (grid-based, per-pixel) - reverted, needs rethink
- Updated AGENTS.md for next session

web/index.html

@@ -6,7 +6,7 @@
   <title>AtomicEngine</title>
   <style>
     * { margin: 0; padding: 0; box-sizing: border-box; }
-    html, body { width: 100%; height: 100%; overflow: hidden; background: #000; }
+    html, body { width: 100%; height: 100%; overflow: hidden; background: #508cdc; }
     canvas {
       display: block;
       position: absolute;

web/src/renderer.ts

@@ -84,7 +84,7 @@ export class Renderer {
 
   render() {
     const gl = this.gl;
-    gl.clearColor(0.0, 0.0, 0.0, 1.0);
+    gl.clearColor(0.31, 0.55, 0.86, 1.0);
     gl.clear(gl.COLOR_BUFFER_BIT);
     gl.useProgram(this.program);
     gl.bindVertexArray(this.vao);