Fix non-terminating Newton iterations in erfinv/erfcinv for BigFloat

[devmotion]

Fixes #557.

The iteration has two representation floors, but only one was tested: the correction falling below the resolution of x. The residual erfc(x) - y cannot go below the resolution of erfc(x), and that noise is amplified into the step by 1 / erfc'(x) = sqrtπhalf * exp(x^2). Whenever sqrtπhalf * exp(x^2) * eps(y) > 2eps(x) the tested condition is unattainable and the iterates end up in a two-cycle. So the residual is now checked as well.

This affects roughly 1–3% of random arguments at every precision. erfinv is affected in the same way for y close to ±1 (erfinv(big"0.96") hangs on master), so it is fixed too.

2eps(y) rather than eps(y): when erfc(x) sits one binade above y, the smallest attainable nonzero residual is already 2eps(y).

Verified with 25000 inversions over precisions 64/128/256/512/1024 in four regimes (mid-range, y → ±1, y → 0, y → 2): no hangs, round-trips within 8eps(BigFloat), and accuracy unchanged against a precision + 128 reference.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.67%. Comparing base (b2a7190) to head (c1d35f9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #559   +/-   ##
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  Coverage   94.67%   94.67%           
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  Files          14       14           
  Lines        3023     3025    +2     
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+ Hits         2862     2864    +2     
  Misses        161      161           

Flag	Coverage Δ
unittests	94.67% <100.00%> (+<0.01%)	⬆️

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