Replace tree/vector atan2 kernels with slope-linear tables, add generated tables and new benchmarks

[falseywinchnet]

Motivation

• Replace the ad-hoc tree/vectoring atan2 codepaths with a compact slope-linear table approach to simplify the hot path and ship precomputed, high-precision constants for speed and reproducible accuracy.
• Add end-to-end audio mag/phase roundtrip and updated atan2 benchmark examples to validate and measure the new mag/phase and phase-extraction code.
• Keep compatibility with legacy ad-hoc build commands by providing a tiny shim for src/atan_vectoring_network_experiment.cpp that delegates to the updated example source.

Description

• Reworked phase extraction internals in src/detail/bruun_kernel.hpp to use templated bruun_phase_slope_leaf and generated slope->leaf tables, introduced bruun_phase_first_octant* and bruun_phase_atan2_mag* entry points, and replaced many previous tree/vector implementations with table-driven code and small inline helpers; added float and double fused helpers and inlining hints.
• Added checked-in generated data headers src/detail/generated/bruun_phase_slope_tables.hpp and src/detail/generated/bruun_sincos_table256.hpp and a generator script tools/gen_phase_slope_tables.py that uses mpmath for regeneration (mpmath is not a build dependency).
• Updated examples and build rules: changed examples/atan2_benchmark.cpp to use the slope-linear path and sample-phase bookkeeping, added examples/audio_magphase_roundtrip_benchmark.cpp, and registered new example executables in CMakeLists.txt; replaced src/atan_vectoring_network_experiment.cpp with a tiny shim that includes the example for backward-compatibility.
• Adjusted tests to reflect the new accuracy target and new checks: loosened mag/phase tolerance in tests/api_c.c, and added check_slope_phase_accuracy and inclusion of bruun_kernel.hpp in tests/correctness.cpp.

Testing

• Built the project with CMake (cmake -B build -S . && cmake --build build -j) and confirmed the build succeeds.
• Ran the test suite with CTest (ctest --test-dir build) and the correctness and api_c tests completed successfully.
• Built and ran the new examples to smoke-check behavior (examples/atan2_benchmark and examples/audio_magphase_roundtrip_benchmark) and observed expected runtime output (benchmarks run to completion).

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Codex Task