[agourakis82]

Complex acosh selects the wrong branch for inputs in the lower half-plane. For example, (2, -1) produces approximately (-1.46935, 0.507356) instead of (1.46935, -0.507356).

Construct the result from the nonnegative imaginary magnitude of acos(z) and apply the sign of the input imaginary component to the output imaginary component. Add mixed-lane float/double coverage in both half-planes, checking the scalar C++ reference, branch signs, and recovery through cosh.

Validation:

• Apple Clang, C++23, ARM64: the identical new test fails before the fix (192 assertions), then the full complex-hyperbolic test unit passes (4 cases, 576 assertions).
• ASan/UBSan and -ffast-math: the same test unit passes.
• Native GCC 13.3, C++23, SSE2 and AVX2: before/after regressions confirmed; the complete complex-hyperbolic test unit passes on both.
• The unchanged original issue reproducer now passes its three nonzero-negative-imaginary acosh examples.
• Formatting and whitespace checks pass.

Addresses the lower-half-plane acosh examples in #1432. The real-axis magnitude defect inherited from acos, other inverse-function defects, nonfinite inputs, and complete architecture coverage remain outside this patch. No claim that #1432 is fully resolved.