Add SDK issued asset payment design

[El-swaggerito]

Description

Difficulty: Expert

Type: Feature

Summary

Design SDK support for issued asset payments.

Current Behaviour

Payment helpers may currently focus on native XLM payments only.

Expected Behaviour

The SDK should have a clear design for supporting issued asset payments without breaking existing native payment helpers.

Suggested Implementation

Create a design or implementation plan covering asset identifiers, trustline checks, payment operation construction, validation, and error handling.

Files or Areas Likely Affected

src/payments/
src/transactions/
src/types/
docs/

Acceptance Criteria

• Issued asset payment support is designed.
• Trustline requirements are documented.
• Validation requirements are documented.
• Native XLM payment behaviour remains supported.
• Tests or examples show issued asset payment inputs.

Additional Notes

This may be documentation-first if full implementation is too large.

[DavidAkere204]

I would like to work on this issue

[grantfox-oss]

🦊 GrantFox — @DavidAkere204 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #165)
2. Your PR will be reviewed by the Axionvera maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@DavidAkere204's PR #201 was approved and merged by @El-swaggerito.

🏆 @DavidAkere204: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (133 total points). Track your full progress on GrantFox.

👏 Great work, @DavidAkere204! Keep contributing to Axionvera.