Add Stellar SDK version compatibility documentation

[meshackyaro]

Description

SDK has peer dependency on stellar-sdk but compatible version range not documented clearly.

Component

SDK

Difficulty

🟢 Easy

Tasks

• Test with different stellar-sdk versions
• Document compatible version range
• Update peer dependency spec
• Add compatibility matrix to README
• CI test against multiple versions

Acceptance Criteria

• Compatible stellar-sdk versions documented
• CI validates compatibility
• Clear upgrade guidance

Estimated Time

1 day