gate cactus on FP16 to prevent SIGILL on older arm64 CPUs

[jakmro]

libcactus.so uses FP16 SIMD. Older arm64 CPUs (SD835, etc.) SIGILL on first inference. Detect support via getauxval(HWCAP_ASIMDHP) and fall back to cloud STT when missing.

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✅ crc-32
❌ jakmro
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