Add hv_approx_fpras() in R (FPRAS hypervolume approximation)

[Copilot]

Ports hv_approx_fpras() from PR #70 to R. The function implements Bringmann & Friedrich's FPRAS algorithm, returning with probability (1-δ) an ε-approximation of the hypervolume — unlike the Monte-Carlo methods in hv_approx(), which take a fixed sample count.

x <- matrix(c(5, 5, 4, 6, 2, 7, 7, 4), ncol = 2, byrow = TRUE)
hypervolume(x, ref = 10)          # 38.0 (exact)
hv_approx_fpras(x, ref = 10, epsilon = 0.01, delta = 0.1)  # ≈38.0

Runtime scales as O(nm/ε² · log(1/δ)) — halving epsilon quadruples runtime.

Changes

• r/R/hv_approx.R — hv_approx_fpras() with roxygen2 docs (math, @warning, @seealso, @references, doctests)
• r/src/Rmoocore.c + r/src/init.h — C wrapper hv_approx_fpras_C (seed, epsilon, delta)
• r/NAMESPACE — export new function
• r/tests/testthat/test-hv_approx.R — error tests and accuracy loop over dims 3–10
• r/vignettes/articles/hv_approx.Rmd — new FPRAS section: error vs. epsilon boxplot and CPU time vs. epsilon line plot (ported from plot_hv_approx.py)
• r/inst/REFERENCES.bib — add Vose1991alias (Walker-Vose alias method)
• r/NEWS.md — entry for new function
• c/hvapprox.c — add m == 0 guard before division to prevent undefined behaviour when budget is exhausted before any dominating pair is found
• c/rng.h — fix DOI (10.1145/3230636)
• c/hvapprox.h — fix spurious comma in comment

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Codecov Report

❌ Patch coverage is 83.83234% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.32%. Comparing base (6e7ab00) to head (bc5d317).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #77      +/-   ##
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+ Coverage   77.13%   77.32%   +0.18%     
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  Files          69       70       +1     
  Lines        7195     7364     +169     
  Branches     1434     1465      +31     
==========================================
+ Hits         5550     5694     +144     
- Misses       1277     1298      +21     
- Partials      368      372       +4     

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Components	Coverage Δ
C	73.30% <81.02%> (+0.26%)	⬆️
R	81.50% <96.66%> (+0.57%)	⬆️
Python tests	100.00% <ø> (ø)
Python src	81.89% <ø> (ø)

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[MLopez-Ibanez]

@copilot You need to run roxygen2 and doctest to generate the appropriate .Rd documentation and .R test file. Otherwise the CI tests will fail.

[Copilot]

@copilot You need to run roxygen2 and doctest to generate the appropriate .Rd documentation and .R test file. Otherwise the CI tests will fail.

Done in commit Fix test, generate hv_approx_fpras.Rd, update doctest file. Since R is not available in the sandbox, I generated the files manually:

• r/man/hv_approx_fpras.Rd — created with the correct roxygen2 structure (description, details, arguments inherited from hv_approx, \section{Warning}, seealso, references, examples).
• r/tests/testthat/test-doctest-hv_approx.R — added the "Doctest: hv_approx_fpras" test block matching the @doctest annotations in R/hv_approx.R.
• r/tests/testthat/test-hv_approx.R line 16 — also fixed the failing CI test: matrix(c(0,0)) with reference=c(1,1) has HV=1 (not 0), so changed to matrix(c(2,2)) which correctly gives HV=0.

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