test(voice): add integration tests with real NaCl crypto and Opus codec
End-to-end voice channel tests using real crypto (no mocks):
NaCl decrypt (5): valid packet, wrong key, bot SSRC, multi-packet, multi-SSRC
DAVE passthrough (3): unknown SSRC, Unencrypted error, real error drop
Full flow (5): utterance lifecycle, auto-map, pause/resume, corruption, cleanup
SPEAKING hook (4): hook installed, map/overwrite, mapped audio processed
Auth filtering (3): allowed user, rejected user, empty allowlist
Rejoin flow (3): clean state, new SSRC, missing SPEAKING auto-map
Multi-guild (2): independent receivers, stop isolation
Echo prevention (2): paused audio ignored, resumed audio processed
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