Sandbox guide

APP_ENV dev or uat acts as SANDBOX; there is no sandbox value. [BT]
A client with the sandbox:control scope is required. [BT]
SCN markets that run: FILL, FOK-REJECT, SLOW-FINAL, EXEC-UNKNOWN, FAIL-AFTER-MATCH, SELL-PARTIAL, PRICE-MOVE. [BT]
SCN markets STOP and PROMO are listed but do not run. [NB]
/v1/s2s/sandbox/*: close, dispute, resolve, correct, copy, reset. [BT]
POST /v1/s2s/event-subscriptions/{id}/test sends one signed test event. [FO] See events.
Wallet keys npwq_ and npws_ for SANDBOX are made in the Back Office (ADR-064). [BT]
Client auth in SANDBOX: client_secret_basic. [FO] INTEGRATION_CLIENT_SECRET_BASIC_ENABLED.

Wallet retry cadence (Neun calls your wallet)

Documented in wallet-v1.json on branch feat/v20-r15-wallet (on a branch, not released). Only a signed HTTP 200 with a status is final. Anything else is retried, except one case: a signed 409 IDEMPOTENCY_CONFLICT stops the retries and opens a recovery case.

Operation	What Neun does today
DEBIT	Retries for a 30 s window, then sends a ROLLBACK.
ROLLBACK	Retries every 60 s (`RELEASE_RETRY_MS`).
REFUND (buy)	Retries every 60 s (`RELEASE_RETRY_MS`).
CREDIT (sell)	Retries every 60 s. A recovery case opens after 5 attempts.
CREDIT (settlement)	Follows the run backoff: 30 s, rising to a cap of 300 s.

ADR-059 s10.3 says the target is 1, 5 and 30 s. The code does not do that yet. Open: N183.