2.4.0 — audio-loop verification

Round-trip audio-loop verification lands as first-class capability: the normalization-aware comparison (soundcheck compare), the 16-fixture canonical audio corpus (soundcheck fixtures check|roundtrip|generate), a content-comparing validate --tts, the new spoken_matches_text gate (11 gates total), an opt-in smart-format transcription option, a nightly recognition-drift check, and a key-resolution fix (an explicitly empty DEEPGRAM_API_KEY no longer falls back to .env files). 239 offline tests. See CHANGELOG.md for the full record. Published to npm via trusted publishing with provenance.