Delay mag-phase conversion and wire atan benchmark

[falseywinchnet]

Motivation

• Move phase computation out of the residue packing loop so residues are first rearranged/packed into standard complex bin order and then converted contiguously to magnitude/phase to enable a more vector-friendly conversion pass.
• Use the existing vec5 cubic/vectorized phase path where possible to accelerate paired phase computation for both double and float.
• Adjust the atan2 microbenchmark and wiring so the benchmark exercises the paired/vectorized path and can be built via make benchmarks for fairer comparisons.

Description

• Reworked forward_mag_phase to first pack Bruun residues into standard complex bin order (fill output[k].re/output[k].im from work) and then call a contiguous conversion helper convert_standard_complex_to_mag_phase. (See src/detail/bruun_kernel.hpp.)
• Added convert_standard_complex_to_mag_phase and convert_standard_complex_to_mag_phase_f32 helpers that perform contiguous complex->(magnitude,phase) conversion and reuse the vec5 cubic paired/vectorized phase path via bruun_phase_atan2_mag_pair for paired lanes where BRUUN_LEVEL >= 1.
• Changed single-precision forward_mag_phase_f32 to pack residues in 4-wide batches using pack4_residues_to_complex_f32 when possible, then call the contiguous convert_standard_complex_to_mag_phase_f32 helper.
• Updated examples/atan2_benchmark.cpp to introduce time_vec5_pair_loop which benchmarks the paired vec5 path, replaced the previous single-sample vec5 timing with the paired/vectorized timing for doubles, and clarified printed labels.
• Wired the new atan2 benchmark into the Makefile by adding an ATAN2_BENCH target and including it in the benchmarks target so it builds with make benchmarks.

Testing

• Ran make test benchmarks which built the library and benchmarks and executed unit probes; build/tests/correctness printed correctness ok and build/tests/api_c printed c api ok indicating success.
• Built and ran the atan2 benchmark build/examples/atan2_benchmark (example run: build/examples/atan2_benchmark 1024), which produced timing output and exercised the paired/vectorized vec5 path; the run completed successfully.
• The benchmarks target reported nothing outstanding to build after changes (make output indicated benchmarks were up-to-date), and compilation of examples succeeded during the test runs.

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Codex Task