Add comprehensive documentation for all game engines

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

docs/engines/unreal_engine/formulas.md

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+# `omath::unreal_engine` — formulas & matrix helpers
+
+> Header: `omath/engines/unreal_engine/formulas.hpp`
+> Namespace: `omath::unreal_engine`
+> Purpose: compute direction vectors, rotation matrices, view matrices, and perspective projections for Unreal Engine
+
+---
+
+## Summary
+
+This header provides **Unreal Engine**-specific math for:
+
+* **Direction vectors** (`forward`, `right`, `up`) from `ViewAngles`
+* **Rotation matrices** from Euler angles
+* **View matrices** (camera transforms)
+* **Perspective projection** matrices
+
+All functions respect Unreal Engine's **Z-up, left-handed** coordinate system.
+
+---
+
+## API
+
+```cpp
+namespace omath::unreal_engine {
+
+  // Compute forward direction from pitch/yaw/roll
+  [[nodiscard]]
+  Vector3<float> forward_vector(const ViewAngles& angles) noexcept;
+
+  // Compute right direction from pitch/yaw/roll
+  [[nodiscard]]
+  Vector3<float> right_vector(const ViewAngles& angles) noexcept;
+
+  // Compute up direction from pitch/yaw/roll
+  [[nodiscard]]
+  Vector3<float> up_vector(const ViewAngles& angles) noexcept;
+
+  // Build 3x3 rotation matrix from angles
+  [[nodiscard]]
+  Mat4X4 rotation_matrix(const ViewAngles& angles) noexcept;
+
+  // Build view matrix (camera space transform)
+  [[nodiscard]]
+  Mat4X4 calc_view_matrix(const ViewAngles& angles,
+                          const Vector3<float>& cam_origin) noexcept;
+
+  // Build perspective projection matrix
+  [[nodiscard]]
+  Mat4X4 calc_perspective_projection_matrix(float field_of_view,
+                                           float aspect_ratio,
+                                           float near, float far) noexcept;
+
+} // namespace omath::unreal_engine
+```
+
+---
+
+## Direction vectors
+
+Given camera angles (pitch/yaw/roll):
+
+* `forward_vector(angles)` → unit vector pointing where the camera looks
+* `right_vector(angles)` → unit vector pointing to the camera's right
+* `up_vector(angles)` → unit vector pointing upward relative to the camera
+
+These are used for movement, aim direction, and building coordinate frames.
+
+---
+
+## Rotation & view matrices
+
+* `rotation_matrix(angles)` → 3×3 (or 4×4) rotation matrix from Euler angles
+* `calc_view_matrix(angles, origin)` → camera view matrix
+
+The view matrix transforms world coordinates into camera space (origin at camera, axes aligned with camera orientation).
+
+---
+
+## Perspective projection
+
+```cpp
+Mat4X4 proj = calc_perspective_projection_matrix(
+  fov_degrees,    // vertical field of view (e.g., 90)
+  aspect_ratio,   // width / height (e.g., 16/9)
+  near_plane,     // e.g., 10.0
+  far_plane       // e.g., 100000.0
+);
+```
+
+Produces a **perspective projection matrix** suitable for 3D rendering pipelines. Combined with the view matrix, this implements the standard camera transform chain.
+
+---
+
+## Usage example
+
+```cpp
+using namespace omath::unreal_engine;
+
+// Camera setup
+ViewAngles angles = {
+  PitchAngle::from_degrees(-20.0f),
+  YawAngle::from_degrees(45.0f),
+  RollAngle::from_degrees(0.0f)
+};
+Vector3<float> cam_pos{1000.0f, 500.0f, 200.0f};
+
+// Compute direction
+auto forward = forward_vector(angles);
+auto right   = right_vector(angles);
+auto up      = up_vector(angles);
+
+// Build matrices
+auto view_mat = calc_view_matrix(angles, cam_pos);
+auto proj_mat = calc_perspective_projection_matrix(90.0f, 16.0f/9.0f, 10.0f, 100000.0f);
+
+// Use view_mat and proj_mat for rendering...
+```
+
+---
+
+## Conventions
+
+* **Angles**: pitch (up/down), yaw (left/right), roll (tilt)
+* **Pitch**: positive = looking up, negative = looking down
+* **Yaw**: increases counter-clockwise from the +X axis
+* **Coordinate system**: Z-up, X-forward, Y-right (left-handed)
+
+---
+
+## See also
+
+* `omath/engines/unreal_engine/constants.hpp` — coordinate frame & angle types
+* `omath/engines/unreal_engine/traits/camera_trait.hpp` — plug-in for generic `Camera`
+* `omath/projection/camera.hpp` — generic camera wrapper using these formulas