Custom asset receive helpers for Stellar (trustline auto-handling)

[truthixify]

Tier: M (2-4 days) Type: feature

Context

Stellar issued assets require trustlines. PR #32 added custom-asset send. Receive needs equivalent helpers (auto-create trustline if absent, warn on auth-required, etc).

Scope

• prepareStealthAccountForAsset(stealthAddress, asset) helper
• Detects whether trustline exists, whether issuer requires auth
• Returns ordered list of ops needed to make the account receivable
• Companion withdrawCustomAsset builder

Acceptance criteria

• Helper + builder + tests
• Documentation
• Demo follow-up for receive UI on issued assets

Files to start with

• sdk/src/chains/stellar/builders.ts
• Stellar trustline docs