Add SDK QR payload parser for payments

[El-swaggerito]

Description

Difficulty: Hard

Type: Feature

Summary

Implement a QR payment payload parser for address, amount, asset, memo, and optional metadata.

Current Behaviour

Mobile payment QR flows may need to parse payloads without duplicating logic outside the SDK.

Expected Behaviour

The SDK should expose a safe QR parser that validates supported payment payload structures.

Suggested Implementation

Create parser types, validation rules, error mapping, payment intent conversion, tests, and documentation.

Files or Areas Likely Affected

• src/payments/
• src/wallet/
• src/types/
• src/errors/
• tests/
• docs/

Acceptance Criteria

• QR payment payload parser is implemented.
• Address, amount, asset, and memo fields are validated.
• Invalid payloads return typed errors.
• Parsed payload can become payment intent input.
• Tests cover valid, invalid, partial, and malformed payloads.
• Documentation explains supported QR format.

Additional Notes

This should support mobile scan-to-pay flows.

[CyberXpert607]

I have reviewed the description and I believe I can work on this if assigned
Thanks

[CyberXpert607]

I have reviewed the description and I believe I can work on this if assigned
Thanks

[grantfox-oss]

🦊 GrantFox — @CyberXpert607 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 #335)
2. Your PR will be reviewed by the Axionvera maintainers

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