docs: append S9-S11, add README and TROUBLESHOOTING

docs/README.md

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+# learn-wgpu
+
+An educational book teaching Rust graphics programming with wgpu 29 + winit 0.30.
+
+Built for backend/systems Rust developers who know Rust thoroughly but have zero
+graphics programming experience. Every sentence teaches GPU concepts — not Rust syntax.
+
+## Required Reading Order
+
+1. [The Graphics Pipeline](concepts/graphics-pipeline.md) — GPU vs CPU, the 5-stage pipeline
+2. [Coordinate Systems](concepts/coordinate-systems.md) — NDC, homogeneous coordinates, viewport
+3. [Shader Basics](concepts/shader-basics.md) — WGSL, vertex/fragment shaders, interpolation
+4. [Build a Rainbow Triangle](01-rainbow-triangle.md) — The complete step-by-step guide
+5. [Troubleshooting](TROUBLESHOOTING.md) — Common errors and fixes (reference as needed)
+6. [Glossary](concepts/GLOSSARY.md) — Every term defined (reference as needed)
+
+> **Reading order matters:** Coordinate systems must be understood before shader basics,
+> because the shader walkthrough references NDC coordinates.
+
+## Guides
+
+- **01 — Rainbow Triangle** — The minimal complete program: one triangle, smooth color interpolation
+- More guides coming soon (textures, lighting, compute)
+
+## Prerequisites
+
+- Rust stable (edition 2024)
+- Linux x86_64 with Vulkan drivers (`libvulkan1`)