Nexus notification sounds.

Files:
- ringtone.mp3      — incoming call ringtone (softphone, IncomingCallModal)
- silent.wav        — keep-alive / audio-unlock helper (ops-center AudioAlertPlayer)
- inbox-message.wav — inbox event "inbound" — new inbound SMS notification
- ding.wav          — inbox event "claim-stolen" — fires when another user claims the conv you have selected (PR-6)
- alarm.wav         — inbox event "ai-escalation" — fires on ai:escalated SSE event (PR-6)
- beep.wav          — reserved / future use

inbox-message.wav is "Taptap" from akx/Notifications
(https://github.com/akx/Notifications), dual-licensed CC0 Public Domain
and CC Attribution 3.0 Unported. Chosen for its short (~260ms), soft,
chat-app feel. Not currently modified.