docs: add Stellar stealth flow diagrams

[pntech20]

Summary

• add five Stellar-native diagrams for stealth flow, key derivation, view-tag scanning, name resolution, and fee-bump withdrawals
• reference the diagrams from the Stellar SDK docs
• add a short Stellar connector note to the architecture overview

Checks

• parsed all new SVGs as XML
• git diff --check

Closes #4

[truthixify]

Heads up — we've just set up a develop branch as the integration target for all Drips × Stellar Wave contributions, and I've retargeted this PR's base from main to develop on your behalf. The diff is unchanged; nothing to do on your end.

Going forward, please branch from and target develop. We'll periodically merge develop → main as we cut releases. Thanks for contributing!