Initial IdealFluid structure#949

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Commits on Feb 16, 2026

Initial IdealFluid structure
mog1el
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Commits on Feb 23, 2026

Added definitions and fixed the properties of earlier defined IdealFluid
mog1el
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Commits on Feb 24, 2026

Imported to PhysLean.lean and added the flow out of the volume
mog1el
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Commits on Feb 26, 2026

pls linters (oh and continuity, isentropy and steadiness)
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