Add storage-layout notes for creator metadata versioning

[Chucks1093]

Summary
Document and formalize how creator metadata can evolve without breaking existing state reads.

Scope

• add versioning notes for creator metadata storage layout
• define compatibility expectations for future metadata fields
• document migration considerations for contributors

Acceptance Criteria

• versioning notes are added in contract docs
• compatibility rules are explicit and reviewable
• notes align with current storage keys and read paths

Contributor coordination: if you pick this up, reach out in Telegram for updates or clarification: https://t.me/accesslayerorg

[Obiajulu-gif]

@Obiajulu-gif has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

My Chief, I would like to work on this issue with you

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Congratulations, @Obiajulu-gif! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @Obiajulu-gif: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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[drips-wave]

This issue has been marked as completed by a Drips Wave moderator for @Obiajulu-gif as part of the Stellar Wave Program's 4th Wave 🥳

Moderator's reason: [Auto-assessed] The PR has been open for over 24 hours without maintainer objections and directly addresses the issue requirements by adding comprehensive documentation on storage layout and versioning in 'docs/storage-evolution.md'. The contributor also included supplementary tests for fee conservation and storage logic, showing a high-quality effort.

😎 @Obiajulu-gif: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here.

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ℹ️ Note: This issue was marked complete via moderation. Points were issued even though the GitHub issue remains open.