feat(FluidDynamics): add pressure field API

[Robby955]

This adds a small pressure-field layer to FluidDynamics, built on the existing FluidState, stress-tensor, and momentum-balance API. It is meant as a starting point for the pressure API discussed in #862; I kept it to the parts the current library supports cleanly, and can adjust the shape to fit the intended requirements.

Added

Physlib/FluidDynamics/Pressure.lean:

• PressureField: a scalar pressure field.
• FluidWithPressure: a FluidState together with a pressure field.
• pressureStress: the isotropic pressure stress -p • I; pressureStress_apply, pressureStress_offDiag, and pressureStress_symm give its components and symmetry.
• pressureForce: the pressure force density -∇p, with pressureForce_apply.
• matrixDiv_pressureStress: the divergence of the pressure stress equals the pressure force, the bridge into the momentum balance.
• toMomentumBalance: packages a FluidWithPressure as momentum-balance data.

Physlib.lean: imports the new module.

Reviewer map

Pressure.lean reads top to bottom: the field and FluidWithPressure, then the stress tensor and its lemmas, then the force and the matrixDiv_pressureStress bridge, and finally toMomentumBalance.

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[jstoobysmith]

See the changes and discussion here #1125 related to setting up Fluid mechanics properly. Maybe if you want to go in this direction you can adapt some of the work of @FloWsnr there (adding them as a co-author on the relevant commits).

[Robby955]

Yes sure, I will study #1125 and I am interested to see if my approach even makes sense with the discussion and @FloWsnr expert work.

[Robby955]

I will read #1125 and @FloWsnr's fluid-mechanics setup and align the pressure layer with that direction rather than the standalone shape here.

[Robby955]

closing this as I believe @FloWsnr approach already has this covered properly