Add documentation for collision detection and mesh classes

Co-authored-by: orange-cpp <59374393+orange-cpp@users.noreply.github.com>

docs/engines/unreal_engine/mesh_trait.md

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+# `omath::unreal_engine::MeshTrait` — mesh transformation trait for Unreal Engine
+
+> Header: `omath/engines/unreal_engine/traits/mesh_trait.hpp`
+> Namespace: `omath::unreal_engine`
+> Purpose: provide Unreal Engine-specific rotation matrix computation for `omath::primitives::Mesh`
+
+---
+
+## Summary
+
+`MeshTrait` is a trait class that provides the `rotation_matrix` function for transforming meshes in Unreal Engine's coordinate system. It serves as a template parameter to `omath::primitives::Mesh`, enabling engine-specific rotation behavior.
+
+---
+
+## Coordinate System
+
+**Unreal Engine** uses:
+* **Up axis**: +Z
+* **Forward axis**: +X
+* **Right axis**: +Y
+* **Handedness**: Left-handed
+* **Rotation order**: Roll (Y) → Pitch (X) → Yaw (Z)
+
+---
+
+## API
+
+```cpp
+namespace omath::unreal_engine {
+
+class MeshTrait final {
+public:
+    [[nodiscard]]
+    static Mat4X4 rotation_matrix(const ViewAngles& rotation);
+};
+
+} // namespace omath::unreal_engine
+```
+
+---
+
+## Method: `rotation_matrix`
+
+```cpp
+static Mat4X4 rotation_matrix(const ViewAngles& rotation);
+```
+
+Computes a 4×4 rotation matrix from Unreal-style Euler angles.
+
+**Parameters**:
+* `rotation` — `ViewAngles` containing pitch, yaw, and roll angles
+
+**Returns**: 4×4 rotation matrix suitable for mesh transformation
+
+**Implementation**: Delegates to `unreal_engine::rotation_matrix(rotation)` defined in `formulas.hpp`.
+
+---
+
+## Usage
+
+### With Mesh
+
+```cpp
+using namespace omath::unreal_engine;
+
+// Create mesh (MeshTrait is used automatically)
+Mesh my_mesh(vertices, indices);
+
+// Set rotation using ViewAngles
+ViewAngles angles;
+angles.pitch = PitchAngle::from_degrees(30.0f);
+angles.yaw = YawAngle::from_degrees(45.0f);
+angles.roll = RollAngle::from_degrees(0.0f);
+
+my_mesh.set_rotation(angles);
+
+// The rotation matrix is computed using MeshTrait::rotation_matrix
+auto matrix = my_mesh.get_to_world_matrix();
+```
+
+---
+
+## Rotation Conventions
+
+Unreal uses a left-handed Z-up coordinate system:
+
+1. **Roll** (rotation around Y-axis / right axis)
+   * Positive roll rotates forward axis upward
+   * Range: [-180°, 180°]
+
+2. **Pitch** (rotation around X-axis / forward axis)
+   * Positive pitch looks upward
+   * Range: typically [-89°, 89°]
+
+3. **Yaw** (rotation around Z-axis / up axis)
+   * Positive yaw rotates clockwise when viewed from above (left-handed)
+   * Range: [-180°, 180°]
+
+**Note**: Unreal applies rotations in Roll-Pitch-Yaw order, different from most other engines.
+
+---
+
+## Type Alias
+
+```cpp
+namespace omath::unreal_engine {
+    using Mesh = primitives::Mesh<Mat4X4, ViewAngles, MeshTrait, float>;
+}
+```
+
+---
+
+## See Also
+
+- [Mesh Documentation](../../3d_primitives/mesh.md) - Mesh primitive
+- [Formulas Documentation](formulas.md) - Unreal rotation formula
+- [CameraTrait Documentation](camera_trait.md) - Camera trait
+
+---
+
+*Last updated: 13 Nov 2025*