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.

src/CMakeLists.txt

@@ -98,4 +98,6 @@ target_sources(${PROJECT_NAME}
         gameplay/chunk.cpp
         gameplay/hitbox_manager.cpp
         gameplay/entity.cpp
+        gameplay/systems/physical_system.cpp
+        gameplay/systems/speed_system.cpp
 )