fix(omnivoice): silence -Werror=double-promotion in voice classifiers

[lalalune]

The ubuntu-24-llguidance CI job builds the fused omnivoice tool with -DLLAMA_FATAL_WARNINGS=ON, turning -Wdouble-promotion into a hard error. Five intentional float→double accumulations in the scalar voice-classifier forwards promoted a float operand implicitly:

• voice_speaker.c: DC-offset removal (338), mel mean-centering (373), TDNN stats-pool mean (575)
• voice_diarizer.c: instance-norm mean (117) and variance (121)

Each is made explicit with a (double) cast — identical numerics, warning gone. The sibling conv/linear += accumulators are float (no promotion) and are correctly left untouched.

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