ADR-0017: Custodial-web wallet

This wallet is the reference implementation of the wallet shape introduced in ADR-0017. The full ADR text lives in the protocol repository at adr/0017-custodial-web-wallet.md; the operator obligations are reproduced in §15.12.1 of the protocol specification.

The five operator obligations:

1. No logging of presentation contents.
2. No cross-user correlation for analytics or third-party purposes.
3. A published, versioned logging policy (see /legal/logging-policy).
4. Credential export in a portable format (Phase 3 of this implementation).
5. Operational distinctness from any specific issuer or relying party in the user's flow.