feat: add Material Forge 3D screenshot to README hero section

Adds the Procedural Material Forge screenshot showing all 8 animated
3D materials with PBR lighting as a hero image right below the
description. Also adds Material Forge link to the top nav.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: web3dev1337

README.md

@@ -2,7 +2,11 @@
 
 Professional WebGL procedural texture generator. Multi-layer compositing, PBR map generation, 3D material preview, 52 custom GLSL shaders, and 65+ procedural effects — all running in your browser with zero dependencies.
 
-**[Live Showcase](https://web3dev1337.github.io/shaderbrew/showcase.html)** | **[Open Editor](https://web3dev1337.github.io/shaderbrew/editor.html)**
+**[Live Showcase](https://web3dev1337.github.io/shaderbrew/showcase.html)** | **[Open Editor](https://web3dev1337.github.io/shaderbrew/editor.html)** | **[Material Forge](https://web3dev1337.github.io/shaderbrew/demos.html)**
+
+<p align="center">
+<img src="docs/media/material-forge-3d.png" width="800" alt="Procedural Material Forge — 8 animated 3D materials with PBR lighting">
+</p>
 
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