refactor(gameplay): extract physics and collision from ClientPlayer into PhysicalSystem

Move per-axis collision detection and move distance calculation to new
PhysicalSystem. Move SpeedSystem implementation from inline header to
separate .cpp file. Add const accessors to Entity. Replace inline AABB
helper with HitboxManager registration of player hitbox using new
PLAYER_SIZE constant. Remove obsolete members and functions from
ClientPlayer.

include/Cubed/gameplay/systems/physical_system.hpp

@@ -0,0 +1,16 @@
+#pragma once
+
+#include "Cubed/gameplay/entity.hpp"
+#include "Cubed/gameplay/world.hpp"
+namespace Cubed {
+class PhysicalSystem {
+public:
+    static glm::vec3 get_move_distance(float dt, const Entity& e);
+    static std::tuple<bool, bool, bool> update(float dt, Entity& e,
+                                               World& world);
+    static std::tuple<bool, bool, bool>
+    update(float dt, Entity& e, World& world, glm::vec3& moved_pos);
+
+private:
+};
+} // namespace Cubed