Fix active speech ending on background noise

[rogerchappel]

Summary

• add a deterministic regression test covering an active utterance followed by sustained sub-speech noise at the configured noise floor
• evaluate below-speech samples against the active utterance silence window before applying the idle noise gate
• clarify active-speech silence, idle noise gating, and event-driven cooldown expiry in the state-machine documentation

Commits

• test: cover active speech ending on background noise
• fix: let background noise close active speech
• docs: clarify silence and idle noise gating

Verification

• npm ci
• npm run release:check (40 tests passed; typecheck, build, smoke, package metadata, install docs, documented imports, and dry-run pack passed)
• git diff --check
• verified every proposed commit author and committer is Roger Chappel <miscanalysis@gmail.com>