Conditioning: carry the conditioning eltype (BigFloat flows end-to-end)

[thisiscam]

#368 loosened condition() to accept any <:Real conditioning point, but the ConditionalCopula and DistortionFromCop structs still stored uⱼₛ/den as Float64 — so a BigFloat (or Float32, Double64, …) point is silently downcast to Float64, and the conditional CDF can't carry that precision.

This parametrises both structs on a value type T, inferred from the conditioning eltype:

• DistortionFromCop{TC,p} → DistortionFromCop{TC,p,T} — fields uⱼₛ::NTuple{p,T}, den::T
• ConditionalCopula{d,D,p,TDs} → ConditionalCopula{d,D,p,T,TDs} — same

The algorithms (_assemble, _swap, _partial_cdf, _process_tuples) are already eltype-generic, so the struct fields were the only thing pinning precision. Backward-compatible: Float64 inputs give T=Float64, behaviour byte-identical.

Result: precision flows through den and the conditional CDF end-to-end wherever the parent copula's pdf/cdf support it (e.g. Clayton/Archimedean). It's also what high-precision nested-Archimedean censored likelihoods need.

Adds a test asserting the conditional CDF is BigFloat-typed (not merely accepted — which #368 already covered), for both the single-distortion and ConditionalCopula paths.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.27%. Comparing base (73649fe) to head (c67c576).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #370      +/-   ##
==========================================
- Coverage   78.28%   78.27%   -0.01%     
==========================================
  Files          84       84              
  Lines        5106     5104       -2     
==========================================
- Hits         3997     3995       -2     
  Misses       1109     1109              

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