feat: implement Stellar custom-asset (issued token) send builder

[bbkenny]

This PR implements a dedicated transaction builder for Stellar stealth payments, with specific support for custom assets (issued tokens).

Key changes:

• Created src/chains/stellar/builders.ts with buildStealthPayment and buildStealthAnnouncement functions.
• Support for native XLM via CreateAccount or Payment.
• Support for custom assets (issued tokens) via CreateClaimableBalance. This bypasses trustline requirements for stealth addresses.
• Added comprehensive unit tests in test/chains/stellar/builders.test.ts.

These builders simplify the process of sending stealth payments on Stellar and ensure consistency across the SDK.

Closes #7

[bbkenny]

Hi @wraith-protocol, I've implemented the transaction builders for Stellar stealth payments as requested in issue #7. These builders include support for native XLM and custom issued assets via Claimable Balances, ensuring stealth addresses can receive tokens without pre-existing trustlines. I've also added comprehensive unit tests. Ready for review!

[drips-wave]

@bbkenny Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[truthixify]

The builder (src/chains/stellar/builders.ts) is the actual deliverable and that part is well-shaped. Two blockers before merge:

1. Rebase against develop. Same pattern as several other PRs this round, your branch was opened against an older main so the diff currently removes work that just landed (CONTRIBUTING.md, the JSDoc additions, the range-scan fetch APIs, the existing announcements test).

   git fetch origin
   git rebase origin/develop
   git push --force-with-lease

2. Drop package-lock.json. This repo uses pnpm and ships pnpm-lock.yaml. The 4931-line package-lock.json would conflict with our package manager. Run:

   rm package-lock.json
   echo "package-lock.json" >> .gitignore  # if not already
   pnpm install  # to regenerate pnpm-lock.yaml against your changes

Once rebased and the lockfile situation is sorted I'll re-review. Thanks @bbkenny.