feat(Cosmology): barotropic equation of state, density scaling laws, cosmological constant as a fluid

[KevorkianPhilippe]

AI disclosure

AI disclosure. This PR was generated with Claude Fable 5.1 (Claude Code) under my
supervision, following AI-POLICY.md and AGENTS.md. I have read every definition and lemma
statement and vouch that they state what the docstrings say. The statements were fixed before
the proofs were written and checked symbolically (sympy) against the Friedmann equations of
Physlib.Cosmology.FLRW.Basic; no textbook is cited, the TODO items of the file are the
specification.

Stacked on #1635 (flrw-matter-4): the diff includes the earlier commits of this series; only the last commit is new here.

Resolves the equation-of-state, scaling-law, specialisation and cosmological-constant TODOs of
MatterContent.lean; the stress-energy tensor TODO stays (no FLRW metric in Physlib).

Declarations added:

• barotropicPressure: p = w ρ c²; deriv_of_continuityEquation_barotropic:
  ∂ₜ ρ = -3 (1 + w) H ρ.
• density_scaling: ρ t = ρ t₀ (a t / a t₀)^(-3(1+w)) (the curve ρ a^(3(1+w)) has zero
  derivative under the barotropic continuity equation at all times, then
  is_const_of_deriv_eq_zero; needs a, ρ differentiable everywhere and a > 0); continuityEquation_of_scaling: the converse.
• density_scaling_dust, density_scaling_radiation, density_scaling_vacuum:
  ρ ∝ a⁻³, ρ ∝ a⁻⁴, ρ constant.
• cosmologicalConstantDensity (moved here from Solutions.lean, which now imports
  MatterContent), cosmologicalConstantPressure; firstOrderFriedmann_iff_lambdaFluid,
  secondOrderFriedmann_iff_lambdaFluid: the Friedmann equations with Λ are those without
  Λ for the fluid ρ + ρ_Λ, p + p_Λ; cosmologicalConstantPressure_eq_barotropic:
  p_Λ is the w = -1 barotropic pressure of ρ_Λ.

Reviewer map: C.1, then C.2 (density_scaling and its converse), C.3, then D.

Update (Sept 11). Applied the review remarks of #1642 across the series: no isolated general-purpose lemma (the one-off HasDerivAt and rpow helpers are inlined in the proofs that used them), and the three small bridges between Time.deriv and Mathlib's HasDerivAt now live next to the definition of the time derivative, in Physlib/SpaceAndTime/Time/Derivatives.lean (deriv_comp_val, deriv_eq_of_hasDerivAt, hasDerivAt_mk_of_differentiableAt, each introduced by the first PR of the series that needs it). Section letters shifted accordingly.

Update (Sept 17). Merged the review of #1635 (statements at t : Time, Time.deriv_comp_val dropped in favour of Time.deriv_eq_of_hasDerivAt); API-map.yaml now records what this PR completes. The one-off helper hasDerivAt_mul_rpow_of_continuityEquation is inlined into density_scaling.

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