Add SDK transaction fee estimation design

[El-swaggerito]

Description

Difficulty: Expert

Type: Feature

Summary

Design transaction fee estimation support for SDK consumers.

Current Behaviour

Consumers may need to show users an estimated fee before signing or submitting a transaction.

Expected Behaviour

The SDK should define how fee estimates are obtained, represented, and displayed safely.

Suggested Implementation

Create a design or helper for fee estimation, fallback behaviour, and uncertainty handling.

Files or Areas Likely Affected

src/transactions/
src/network/
src/types/
docs/

Acceptance Criteria

• Fee estimation behaviour is designed.
• Fallback behaviour is documented.
• Fee estimate result type is defined.
• Uncertainty is represented clearly.
• Examples are included.

Additional Notes

Do not guarantee exact fees unless the SDK can support that.

[adnaan-id]

Can I try this one, maintainer? I can submit a pr in 2 days

[grantfox-oss]

🦊 GrantFox — @adnaan-id 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 #239)
2. Your PR will be reviewed by the Axionvera maintainers

Good luck! Track your progress on GrantFox.