Add Parametric EQ

[PEQHUB]

Summary

• Added a full parametric equalizer with peaking, low shelf, and high shelf filter types
• Biquad-based filter computation with real-time frequency response preview
• Preamp control (0 to -30 dB) to prevent clipping when boosting
• Import/export support for EqualizerAPO format (including preamp)
• PEQ output merges with the existing graphic EQ via log-linear interpolation

[ckuma]

FWIW I've built your branch with no issues and could confirm the import of text files fitting the Peace format (not the format of Peace "exports", but indeed the actual filters from eqapo peace.txt stuff).
The feature is quite helpful, thanks!

Example of an importable file (as text):

Preamp: -4.4 dB
Filter 1: ON PK Fc 94 Hz Gain 2.2 dB Q 2.2
Filter 2: ON LSC Fc 105 Hz Gain -6.6 dB Q 0.7
Filter 3: ON PK Fc 157 Hz Gain -1.2 dB Q 2.45
Filter 4: ON PK Fc 446 Hz Gain 2.7 dB Q 2.45
Filter 5: ON PK Fc 1072 Hz Gain -1.4 dB Q 3.66
Filter 6: ON PK Fc 1716 Hz Gain 3 dB Q 4.34
Filter 7: ON PK Fc 2910 Hz Gain 2.2 dB Q 3.34
Filter 8: ON PK Fc 9279 Hz Gain -2.3 dB Q 2.9
Filter 9: ON PK Fc 9687 Hz Gain -2.2 dB Q 0.6
Filter 10: ON HSC Fc 10000 Hz Gain 4.5 dB Q 0.7

(In this case a tweaked EQ for the Momentum 4 wireless headphones)

A note of comparison: the app Wavelet has ways to import multiple EQs into their own preset, allowing for easier toggling between imports which might be fitting to retain more imports with easy switching.
A single-select (radiobox) list with inline delete would avoid having to file-pick the files every time. I haven't touched android dev in nearly a decade but might look into that based on your work, because your PR just works as-is already :)

(Phone tested: Samsung Galaxy S22 ultra, Android 16)