Re-implements envelope detection on top of the rewritten Morse decoder.
Addresses PR #160 review feedback:
- Rebase: rebuilt on current upstream/main (lifecycle state machine)
- Gap thresholds: 2.0/5.0 for envelope only; goertzel keeps 2.6/6.0
- Frequency validation: max_mhz=1766 for envelope, 30 for goertzel
- Tests: EnvelopeDetector unit tests + envelope-mode decoder test
- Envelope uses direct magnitude threshold (no SNR/noise ref)
- Goertzel path completely unchanged
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>