feat(stellar): add batch transaction builder

[pharwasz]

Closes #139

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## Summary

Implements a Stellar batch transaction builder for stealth payments.

Changes

• Added buildBatchSendTx() for constructing multi-recipient Stellar transactions.

• Added fee scaling based on operation count.

• Added configurable operation-count limit with descriptive validation errors.

• Added threshold logic for future stealth-batch-sender contract integration.

• Returns transactions ready for external signature collection.

• Added test coverage for:

  • Single recipient
  • 5 recipients
  • 20 recipients
  • Operation limit validation
  • Threshold crossover behavior

Notes

• The stealth-batch-sender integration is intentionally stubbed until the contract is available.
• The threshold selection logic is implemented and covered by tests.

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[pharwasz]

hi there, can you please confirm this PR and approve