Add helper for formatting a bigint stroop value as a signed XLM delta string for transaction history display

[Chucks1093]

Summary

Transaction history entries show the XLM change per transaction (e.g. +12.5000000 XLM or -3.0000000 XLM). The sign and formatting logic is currently duplicated across buy and sell serialisers. A shared formatXlmDelta(stroops: bigint, direction: 'in' | 'out'): string helper should centralise it.

Scope

• Add formatXlmDelta(stroops, direction) returning a signed string: +X.XXXXXXX XLM for 'in', -X.XXXXXXX XLM for 'out'
• Add unit tests: 10000000 stroops 'in' → '+1.0000000 XLM', 10000000 stroops 'out' → '-1.0000000 XLM', 0 stroops 'in' → '+0.0000000 XLM'
• Replace all inline delta formatting in transaction history serialisers with the helper
• Helper must use bigint arithmetic (no floating point)

Acceptance Criteria

• 'in' direction returns positive prefix
• 'out' direction returns negative prefix
• 0 stroops returns correctly formatted zero
• All existing delta formatting uses the helper
• No floating-point arithmetic

ETA: 12 hours

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Coordinate on Telegram

[akinboyewaSamson]

@akinboyewaSamson has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello maintainer I’d love to help out with this issue. I’ve checked the current codebase/documentation and know exactly what needs to be updated to make this clearer. I will ensure everything aligns perfectly with your existing contribution guidelines.

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[zoffunjunior381-jpg]

@zoffunjunior381-jpg has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi maintainer, I’d love to work on this issue.

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[Namiiikaze]

@Namiiikaze has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I'd like to work on this. Plan: add a formatXlmDelta(stroops, direction) helper returning a signed string, +X.XXXXXXX XLM for 'in' and -X.XXXXXXX XLM for 'out', using bigint arithmetic throughout to avoid floating-point precision issues. Will replace the duplicated inline delta formatting in the buy and sell transaction history serialisers with calls to this helper, and add tests covering 10000000 stroops 'in', 10000000 stroops 'out', and the 0 stroops zero-value case.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Namiiikaze to this issue.

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Congratulations, @zoffunjunior381-jpg! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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[grantfox-oss]

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

@zoffunjunior381-jpg's PR #714 was approved and merged by @Chucks1093.

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👏 Great work, @zoffunjunior381-jpg! Keep contributing to accesslayerorg.

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