Create architecture diagram for split lifecycle

[mimijuwonlo-commits]

Area

Architecture documentation

Problem

The split lifecycle crosses frontend, backend, database, Soroban contract, wallet, and RPC services. A maintained diagram would help contributors reason about changes.

Acceptance Criteria

• Add a diagram covering create, fund, distribute, and observe flows
• Include trust boundaries and external services
• Use a repo-friendly diagram format
• Link from README and architecture docs

Validation

Preview the rendered diagram and verify labels match current components.