Add a dependency-free signal-analysis toolkit and a `specview` dev CLI to
evaluate audio (especially echo cancellation) with objective numbers and a
terminal spectrogram instead of ear alone.
- src/dsp/: hand-written radix-2 FFT, Hann window, STFT, seeded test-signal
generators (sine/log-sweep/white/pink/impulse), metrics (RMS/dBFS/peak/ERLE/
per-band energy), minimal WAV read+write, and a 24-bit-ANSI half-block
spectrogram renderer (magma colormap, freq/time axes, dB legend, ASCII
fallback). Pure layers have no I/O; only `wav` touches the filesystem.
- src/bin/specview.rs: `gen` (conjure a test signal -> WAV), `show` (spectrogram
+ per-band energy summary), `erle` (broadband + per-band echo-return-loss
between a before/after pair).
- 27 unit tests (FFT correctness, ERLE landmarks, WAV round-trip, render shape).
Verified end-to-end: log sweep renders as the expected exponential curve;
a 20 dB-quieter copy reads +20.0 dB ERLE broadband and per band.
Measurement substrate for upcoming AEC refinement (no shipped-path changes).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
molluskandClaude Opus 4.8
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