refactor: split Stellar library into domain modules

[wisdom2030-dotcom]

Summary

Closes #

How was this tested?

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• Tests were added or updated for the change (primary flow, a boundary case, and a failure case).
• Docs were updated where behaviour, configuration, or security posture changed.

Security-sensitive changes

• This PR touches a path covered by .github/CODEOWNERS (wallet, auth, cryptography, CI) and a code owner has been requested for review.
• I described any change to key handling, signing, session lifetime, or trusted endpoints above.

closes #958

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