Replace the O(n) loop in nextafter(x, y, steps) with O(1) IEEE 754
bit manipulation matching math_nextafter_impl in mathmodule.c.
Handles sign-crossing, saturation, and all edge cases.

Add pyo3 proptest and edge/extreme-step tests for nextafter.

cmath: Rewrite log(z, base) to match cmath_log_impl errno
semantics. c_log always returns a value and separately reports
EDOM for zero. c_quot returns EDOM and (0,0) for zero
denominator instead of NaN.

bigint: Add sticky bit to frexp_bigint for correct IEEE
round-half-to-even, matching _PyLong_Frexp. Use mul_add for
log/log2/log10 bigint to match loghelper fma usage.

Add regression tests for remainder tie-to-even and signed zero.

- math_1/math_2: document why errno handling differs from
  CPython (platform-specific unreliability, output checks
  sufficient, verified by proptest)
- math.log: document EDOM substitution for ZeroDivisionError
- math.remainder: document libm delegation rationale
- sumprod: return Result for length mismatch instead of panic,
  improve overflow fallback to continue from where the fast
  path stopped instead of restarting from scratch