nx_polypitch.nx

nx_polypitch.nx -- INSTRUMENT-LEARN-GAME rung LG-3: POLYPHONIC pitch detection (chords). Guitars and pianos play several notes at once; a single-F0 detector (nx_pitch) can't see that. Instead of a big FFT (nx_fft caps at N=32 = 250 Hz bins, useless for notes), we run a GOERTZEL filter bank: evaluate the signal's energy DIRECTLY at each 12-TET note frequency. One cosine per note, O(N) per note -- exact note resolution, no FFT-bin quantization. A note is "present" when its Goertzel power is a local peak across the note grid AND above a fraction of the strongest note's power. The set of present notes = the chord. Pure integer. Imports only the x86_64 syscall layer (mmap scratch). Also exposes a sovereign full-range sin/cos (nx_pp_sinfull / nx_pp_cosfull) reusable across the audio arc. license_tier: ORIGINAL module: nishi-core.audio.polypitch depends: nishi-core.audio.note (note table, caller-built) capability: AUDIO_POLYPHONY