Update bundled vinecopulib to the 1.0 conditional-simulation stack

[tnagler]

Summary

• vendor all headers from vinecopulib@4efce25966871a09f71891183b57082ff5e080e1, the current tip of cond-simulate
• make the source update script location-independent and ref-selectable, preserve package-owned wrapper headers, and stop patching/deleting upstream headers
• require C++17, R >= 4.0.0, and BH >= 1.75.0-0
• select the RcppThread backend from the package-owned wrapper, keeping the vendored upstream headers pristine and exportable

Backend/API changes relevant to the R frontend

The imported stack adds explicit conditioning-set overloads for conditional simulation and Rosenblatt transforms, a per-observation-parameter Bicop::simulate() overload, serial evaluation of custom tree criteria, unified compact/expanded discrete layouts, and the vinecopulib 1.0 API cleanup. Follow-up stack layers consume these APIs; the existing frontend compiles without compatibility shims.

The backend also contains the Tawn override corrections, corrected BB6/BB7/BB8/Tawn Kendall-tau integration, and the final fix that treats omitted pair copulas as implicit independence during evaluation.

Tests

• R CMD INSTALL with C++17
• complete current testthat suite against the installed package
• downstream Rcpp::sourceCpp() smoke test using the installed vinecopulib-wrappers.hpp; constructed a Bicop and verified VINECOPULIB_VERSION == 100000
• verified the vendored upstream header tree byte-for-byte against the pinned backend checkout
• after cascading the final backend refresh through stack #330: 817 testthat checks passed and R CMD check --no-manual --no-build-vignettes reported Status: OK at the release tip

No existing R simulation API is changed in this bottom layer; R-side seed handling is exercised in the conditional and vectorized simulation layers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (044a1c9) to head (251b548).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #328   +/-   ##
=======================================
  Coverage   97.15%   97.15%           
=======================================
  Files          16       16           
  Lines        2001     2001           
=======================================
  Hits         1944     1944           
  Misses         57       57           

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