Neon Nexus crash recovery

Provider rounds retain the existing idempotent recovery contract:

1. Repeating a client_round_id resumes the stored round.
2. Any open round is completed before another wager is accepted.
3. GET /v1/rounds/current exposes an interrupted round after relaunch.
4. Wallet debit and credit reuse durable transaction references.

The play-session token remains memory-only. PlayerPal or widget outages never block launch, wagering, settlement, or recovery. Persisted Activity outbox rows resume independently after provider-service restart.