refactor(gameplay): extract Entity base class from ClientPlayer

Move position, walk pose, velocity, orientation, movement, and gravity
fields and their accessors into a new Entity base class. ClientPlayer now
inherits from Entity, removing duplicated members. Also update velocity
handling to use 3D vector per axis and adjust related logic.

src/gameplay/entity.cpp

@@ -0,0 +1,14 @@
+#include "Cubed/gameplay/entity.hpp"
+
+namespace Cubed {
+glm::vec3& Entity::max_speed() { return m_velocity.max; }
+float& Entity::acceleration() { return m_movement.acceleration; }
+float& Entity::deceleration() { return m_movement.deceleration; }
+float& Entity::g() { return m_gravity.value; }
+void Entity::set_gait(Gait gait) { m_walk_pos.gait = gait; }
+float Entity::yaw() const { return m_angle.yaw; }
+float Entity::pitch() const { return m_angle.pitch; }
+float& Entity::roll() { return m_angle.roll; }
+float& Entity::walk_time() { return m_walk_pos.walk_time; }
+Gait Entity::get_gait() const { return m_walk_pos.gait; }
+} // namespace Cubed