Stellar offline transaction signing helper

[truthixify]

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

Context

Cold storage and air-gapped wallets need to sign Stellar transactions without RPC access. Provide a helper that builds + signs offline, leaving submission as a separate step.

Scope

• prepareOfflineStellarTransaction({ source, ops, sequence, networkPassphrase }) returns serializable envelope
• signOfflineStellarTransaction(envelope, key) returns signed XDR
• submitOfflineStellarTransaction(xdr, network) separate call
• All three documented as a workflow for cold-signing flows
• Stealth-address aware (can sign withdrawals from derived stealth keys)

Acceptance criteria

• Three helpers + tests
• End-to-end test: build offline, sign offline, submit online
• Documentation page describing the cold-signing workflow

Files to start with

• New: sdk/src/chains/stellar/offline-sign.ts