Cover the rest of the sin, cos and tan reduction

[ibmibmibm]

Summary

This PR is a follow-up to #1475. It adds two test points for sin, cos and tan, and it removes three branches of the reduction that no input can reach.

New tests

• test_trig_rounding now tests -10^19. This value uses the reduction with base-1e9 words for a negative argument.
• test_trig_rounding now tests 10^-80. For an argument this small, fx_from gives 0, and the kernel uses the argument as the result.

Removed branches

• to_words for a 128-bit significand now always returns 4 words. frexp10 gives 34 digits, thus the high word is never zero. Zero high words do not change the product.
• The decimal and binary reductions do not test for a zero fraction. The worst cases have at most 37 leading zero digits and 117 leading zero bits, thus the fraction is never zero. A comment at each loop gives these limits.

Tests

GCC 16.2.1 and Clang 22.1.8, C++14 and C++20, with -Wall -Wextra -Werror -Wconversion -Wsign-conversion -Wshadow -Wundef: test_trig_rounding, test_sin_cos, test_tan, test_cmath and test_fenv pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.7%. Comparing base (7c79789) to head (9c582c2).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1481     +/-   ##
=========================================
+ Coverage     98.7%   98.7%   +0.1%     
=========================================
  Files          313     313             
  Lines        26252   26255      +3     
  Branches      2249    2246      -3     
=========================================
+ Hits         25897   25908     +11     
+ Misses         355     347      -8     

Files with missing lines	Coverage Δ
...de/boost/decimal/detail/cmath/impl/trig_reduce.hpp	100.0% <100.0%> (+2.8%)	⬆️
test/test_trig_rounding.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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[ckormanyos]

LGTM. Merging