Fix fast_integer_divide for signed numerator with denominator == 1

[alexreinking]

The signed round-toward-negative-infinity branch of fast_integer_divide rewrites numerator in place (numerator = xsign ^ numerator) to fold in the sign, but the denominator == 1 special case then returned that bit-flipped value instead of the original. So fast_integer_divide(n, 1) returned ~n for negative signed n (e.g. int8 -128 -> 127, -5 -> 4, 0 -> -1).

Capture the original numerator before the branch and use it in the denominator == 1 select. The unsigned and round-to-zero branches never reassigned numerator, so they are unaffected.

Adds test/correctness/fast_integer_divide.cpp, which compares fast_integer_divide/fast_integer_modulo against floor division over u8/u16/u32/s8/s16/s32; it exercises the signed denominator == 1 case that previously went uncovered.

Breaking changes

fast_integer_divide now returns 0 with a zero denominator. fast_integer_modulo now returns 0 with a 0 modulus.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[alexreinking]

This was found and fixed by Opus while working on something else.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.26%. Comparing base (52c8d33) to head (1b41862).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/FastIntegerDivide.cpp	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9271      +/-   ##
==========================================
+ Coverage   70.18%   70.26%   +0.07%     
==========================================
  Files         257      257              
  Lines       79105    79109       +4     
  Branches    18954    18954              
==========================================
+ Hits        55518    55583      +65     
+ Misses      17908    17866      -42     
+ Partials     5679     5660      -19     

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[abadams]

Please fix test/const_division.cpp to include denominators of 1 (and zero) rather than adding a new test.

[alexreinking]

Adding release_notes for the semantic change here.