Feat/physics world improvement

[Satvik-Singh192]

Description

Refactored the Rigidbody implementation and introduced a proper PhysicsWorld class.
Moved integration logic from bodies into the world to keep architecture clean and layered.

This sets up the base structure for gravity and future collision handling.

Changes Made

• Removed self-update logic from Rigidbody
• Added force accumulation system inside Rigidbody
• Introduced PhysicsWorld class to manage bodies
• Centralized gravity and integration inside PhysicsWorld::step
• Cleaned separation between main loop and engine logic

Implementation Details

• Rigidbody now only stores physical state (position, velocity, inverse mass, force accumulator).
• PhysicsWorld owns all bodies and handles integration per timestep.
• Gravity is applied as a force inside the world step.
• main.cpp only handles fixed timestep and calls world.step(dt).

Checklist

Type:

• Bug Fix
• Feature
• Chore