feat: add simulation step controls

[nicolaszmarzagao]

Summary

Closes #319

Adds Step Backward and Step Forward controls next to the Play/Pause button.

The buttons manually inject a fixed simulation delta while paused:

• Step Backward: -0.01
• Step Forward: +0.01

This helps users inspect simulations frame-by-frame without needing to pause at the exact moment.

Changes

• Added a reusable StepButton control.
• Added manual step support to the shared time system.
• Updated simulation dt guards so negative manual steps are not ignored.
• Updated shared physics body stepping to allow non-zero negative deltas.

Testing

• Ran the project locally with npm run dev.
• Confirmed step buttons appear in simulations.
• Tested forward and backward stepping while paused.
• Confirmed Play/Pause and Reset still work.
• Tested across multiple simulations.

[mattqdev]

Hey thank you, it seems really intereseting. Can you send me please a short clip of it works?

[nicolaszmarzagao]

Here is the short clip demonstrating the feature...

https://youtu.be/2WMNygiFMjI

[mattqdev]

Alright thank you very much for this pr!