Improve DSP measurement accuracy

[CryptoJones]

Summary

• improve dominant pitch detection with harmonic stack scoring/refinement
• fold chroma from FFT-bin power with semitone interpolation and key confidence gate
• improve production metering with fractional band energy, octave-density balance, biquad band splits, and 4x intersample peak estimate
• bump version to 3.6.1

Verification

• /usr/bin/gcc -Wall -Wextra -Werror -O2 -std=c99 -o /tmp/test_prod tests/test_prod.c -lm
• /tmp/test_prod
• /usr/bin/gcc -Wall -Wextra -Werror -O2 -std=c99 -o /tmp/test_pitch tests/test_pitch.c -lm
• /tmp/test_pitch
• /usr/bin/gcc -Wall -Wextra -O2 -std=c99 -o /tmp/test_lufs tests/test_lufs.c -lm
• /tmp/test_lufs
• /usr/bin/gcc -Wall -Wextra -shared -fPIC -O2 -DMODULE_STRING="dixwaveform" -D__PLUGIN__ -I/usr/include/vlc/plugins -D_FILE_OFFSET_BITS=64 -D_REENTRANT -D_THREAD_SAFE waveform.c -lvlccore -lm -o /tmp/libdixwaveform_plugin.so
• git diff --check

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