Add comprehensive documentation for all game engines

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

docs/engines/unity_engine/constants.md

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+# `omath::unity_engine` — types & constants
+
+> Header: `omath/engines/unity_engine/constants.hpp`
+> Namespace: `omath::unity_engine`
+> Purpose: define Unity Engine coordinate system, matrix types, and angle ranges
+
+---
+
+## Summary
+
+The **Unity Engine** uses a **Y-up, left-handed** coordinate system:
+
+* **Up** = `{0, 1, 0}` (Y-axis)
+* **Right** = `{1, 0, 0}` (X-axis)
+* **Forward** = `{0, 0, 1}` (Z-axis)
+
+Matrices are **row-major**. Angles are **clamped pitch** (±90°) and **normalized yaw/roll** (±180°).
+
+---
+
+## Constants
+
+```cpp
+namespace omath::unity_engine {
+  constexpr Vector3<float> k_abs_up      = {0, 1, 0};
+  constexpr Vector3<float> k_abs_right   = {1, 0, 0};
+  constexpr Vector3<float> k_abs_forward = {0, 0, 1};
+}
+```
+
+These basis vectors define the engine's **world coordinate frame**.
+
+---
+
+## Matrix types
+
+```cpp
+using Mat4X4 = Mat<4, 4, float, MatStoreType::ROW_MAJOR>;
+using Mat3X3 = Mat<4, 4, float, MatStoreType::ROW_MAJOR>;
+using Mat1X3 = Mat<1, 3, float, MatStoreType::ROW_MAJOR>;
+```
+
+**Row-major** storage means rows are contiguous in memory. Suitable for CPU-side transforms and typical C++ math libraries.
+
+---
+
+## Angle types
+
+```cpp
+using PitchAngle = Angle<float, -90.f, 90.f, AngleFlags::Clamped>;
+using YawAngle   = Angle<float, -180.f, 180.f, AngleFlags::Normalized>;
+using RollAngle  = Angle<float, -180.f, 180.f, AngleFlags::Normalized>;
+
+using ViewAngles = omath::ViewAngles<PitchAngle, YawAngle, RollAngle>;
+```
+
+* **PitchAngle**: clamped to **[-90°, +90°]** (looking down vs. up)
+* **YawAngle**: normalized to **[-180°, +180°]** (horizontal rotation)
+* **RollAngle**: normalized to **[-180°, +180°]** (camera roll)
+
+`ViewAngles` bundles all three into a single type for camera/view transforms.
+
+---
+
+## Coordinate system notes
+
+* **Y-up**: gravity points along `-Y`, height increases with `+Y`
+* **Left-handed**: cross product `forward × right = up` with left-hand rule
+* This matches **Unity Engine** conventions for 3D games and simulations
+
+---
+
+## See also
+
+* `omath/engines/unity_engine/formulas.hpp` — view/projection matrix builders
+* `omath/trigonometry/angle.hpp` — angle normalization & clamping helpers
+* `omath/trigonometry/view_angles.hpp` — generic pitch/yaw/roll wrapper