Add NEON (aarch64) vectorized paths mirroring the existing AVX2 paths:
- demod/math_arm.rs: replace the no-op placeholder with a full NEON FM
discriminator that processes 4 samples per iteration using a 7th-order
minimax atan polynomial and branchless atan2 with argument reduction,
matching the accuracy of the AVX2 path (max error ~2.4e-7 rad).
32-bit ARM retains the scalar fallback.
- dsp/filter.rs: add mul_freq_domain_neon() that deinterleaves 4 complex
pairs via vuzpq/vzipq, performs complex multiply with vmulq/vaddq/vsubq,
then reinterleaves. On aarch64 this path is always taken (NEON is
mandatory); scalar fallback remains for other targets.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>
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