Add storage key registry to prevent key collision across contract versions

[Kingsman-99]

Overview

Storage keys are currently defined ad-hoc as string literals scattered across the codebase. Centralise them in a key registry to prevent collisions and make key migration between contract versions explicit.

Acceptance Criteria

• StorageKey enum covers all persistent, instance, and temporary storage keys used by the contract
• All env.storage() calls use StorageKey variants — no raw string literals remaining
• Key variants namespaced: StorageKey::Invoice(u64), StorageKey::Config, StorageKey::Counter, etc.
• STORAGE_KEY_REGISTRY.md documents every key, its type, its TTL, and which contract version introduced it
• Migration utility function: migrate_key(old_key, new_key) for use in upgrade procedures
• No two StorageKey variants serialize to the same XDR value (verified by test)
• Unit tests assert key uniqueness across all variants

[Adesam007-pr3dator]

@Adesam007-pr3dator has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I would love to work on this issue

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[jerrymusaga]

@jerrymusaga has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Please, I will like to work on this

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[rindicomfort]

@rindicomfort has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello Maintainer,

Please I’d love to take this issue on. I’m a smart contract and full-stack developer with experience across OnlyDust, Drips Waves

My stack includes Rust, Soroban, Cairo, Solidity, and TypeScript. I can deliver a clean, tested, and well-documented solution within 24 hours of assignment.

Happy to start immediately. Thank yOU

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[ndyugwu]

@ndyugwu has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

i would like to work on this

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[drips-wave]

Congratulations, @jerrymusaga! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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