⚙️ Core: Strategy Pattern for Fluid Conservation

[fderuiter]

This PR refactors the fluid dynamics conservation module to adhere to SOLID principles.

🔧 The Violation:
The conservation.rs module contained duplicative functions (navier_stokes_time_derivative, euler_time_derivative) with hardcoded physics logic and long argument lists ("Tuple Soup"), violating OCP and DRY.

🛠️ The Fix:

• Applied the Strategy Pattern by extracting a MomentumEquation trait.
• Implemented concrete strategies NavierStokes (viscous) and Euler (inviscid).
• Introduced a SpatialGradients parameter object to group related arguments.

📉 Debt Reduction:

• Eliminated logic duplication for convection and pressure terms.
• Decoupled physics models from the time-stepping function signature.
• Prepared the module for easy extension (e.g., adding Stokes flow or non-Newtonian models).

🧪 Integrity:

• All tests passed.
• Mathematical results are preserved bit-identically.

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PR created automatically by Jules for task 9202061444194405817 started by @fderuiter

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