FixedPhysics now uses Double instead of Float for increased precision.

[SoloByte]

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[Copilot]

Pull request overview

This PR converts the fixed physics update system from single-precision (float) to double-precision (double) for improved accuracy in physics calculations. The changes affect timing calculations throughout the fixed update loop, including the physics accumulator, frame time calculations, and timestep management.

Changes:

• Added a new DeltaDouble property to GameTime for accessing elapsed time with double precision
• Changed FixedPhysicsTimestep from float to double for more precise timestep calculations
• Updated AdvanceFixedUpdate method to use double throughout physics accumulation and timing logic
• Modified FixedTime to use Tick(double) instead of TickF(float) for consistent double precision

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
ShapeEngine/Core/Structs/GameTime.cs	Adds DeltaDouble property to provide direct access to the underlying ElapsedSeconds value without float conversion
ShapeEngine/Core/GameDef/GameGameloop.cs	Updates AdvanceFixedUpdate to accept and use double for all timing calculations and changes call from TickF to Tick
ShapeEngine/Core/GameDef/Game.cs	Changes FixedPhysicsTimestep property and physicsAccumulator field from float to double

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