Add claude docs

.claude/display-shader.md

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+# Display -- Shader, Material, and Display State
+
+Source: `src/dusk/display/`
+
+See also: `.claude/display-core.md`, `.claude/display-texture.md`
+
+---
+
+## Shader (`shader.h` + `shaderlist.h`)
+
+Shaders are platform-abstracted. The current shader list is defined
+in `shaderlist.h`. Currently only one shader is implemented:
+
+| Enum | Description |
+|------|-------------|
+| `SHADER_LIST_SHADER_UNLIT` | Unlit / flat colour + texture shader |
+
+```c
+extern shaderlistdef_t SHADER_LIST_DEFS[SHADER_LIST_SHADER_COUNT];
+// SHADER_LIST_DEFS[n].shader is the platform shader object.
+
+// Bind a shader before drawing:
+errorret_t shaderBind(shader_t *shader);
+
+// Upload a mat4 uniform by name:
+errorret_t shaderSetMatrix(shader_t *shader, const char_t *name, mat4 m);
+```
+
+Adding a new shader means adding an entry to `shaderlist.h`, providing
+platform-specific vertex/fragment sources, and implementing the
+corresponding material type in `shadermaterial_t`.
+
+---
+
+## Shader material (`shadermaterial.h`)
+
+`shadermaterial_t` is a union over per-shader material structs.
+Currently contains only the unlit material:
+
+```c
+typedef union shadermaterial_u {
+  shaderunlitmaterial_t unlit;
+} shadermaterial_t;
+```
+
+`shaderunlitmaterial_t` typically holds a `texture_t *` and a
+tint colour. Check `shaderunlitmaterial.h` for the exact fields.
+
+To use a shader material on a renderable entity:
+
+1. Set `renderable.type = ENTITY_RENDERABLE_TYPE_SHADER_MATERIAL`.
+2. Set `renderable.data.material.shaderType` to the desired
+   `shaderlistshadertype_t` value (e.g. `SHADER_LIST_SHADER_UNLIT`).
+3. Fill in the corresponding union field:
+   `renderable.data.material.material.unlit`.
+4. Set `renderable.data.material.state.flags` for rasterizer state.
+
+---
+
+## Display state (`displaystate.h`)
+
+`displaystate_t` controls per-draw rasterizer state flags:
+
+```c
+DISPLAY_STATE_FLAG_CULL        // back-face culling
+DISPLAY_STATE_FLAG_DEPTH_TEST  // depth testing
+DISPLAY_STATE_FLAG_BLEND       // alpha blending
+```
+
+Set flags via `data.material.state.flags` on the renderable's material.
+The default for an uninitialised state is all flags clear (no culling,
+no depth test, no blending). Most opaque geometry should set at least
+`CULL | DEPTH_TEST`.