The previous magnitude-based threshold couldn't distinguish CW tone from
AGC-amplified inter-element silence — the Goertzel level stayed above
threshold permanently, preventing any tone OFF transitions and thus zero
character decodes.
Switch tone detection to use SNR (tone_mag / adjacent_band_noise_ref).
Both bands are equally amplified by AGC, so the ratio is gain-invariant.
Also replace the conditional noise_ref guard with unconditional blending
so the noise floor tracks actual ambient levels continuously.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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