Write the storage-archival ADR

[Otaiki1]

Context

Soroban archives storage entries whose TTL lapses, and the contract's behaviour under archival differs
by entry: Circle entries are re-extended on every fund/claim; the NextCircleId instance entry
is extended on create_circle (with a comment explaining why); nullifier entries are written once and
never touched again.

The nullifier case is a security fence, so its lifetime is a security property. That analysis is
scattered across code comments and contracts/README.md and should be one document.

Where to build it

• New file docs/adr/004-storage-archival.md, following docs/adr/001-upgradeability.md's format.
• contracts/README.md — link it and trim the duplicated prose.
• docs/index.md — add to the ADR list.

What to do

• Enumerate every storage entry, its type (instance vs persistent), when its TTL is extended, and the consequence of archival.
• Analyse the nullifier lifetime specifically, including the replay risk.
• Record the decision and its consequences, and link the implementation issue.
• State the operational implication: how long a circle can lie dormant and still be safe.

Acceptance criteria

• Every storage key in DataKey appears in the ADR's table.

[Iszhyie]

@Iszhyie has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello Maintainer,
I've gone through this issue carefully and have a clear understanding of the requirements, expected design, and acceptance criteria. This is a good match for my experience with similar tasks, and I'm confident I can deliver a clean, well-tested solution that meets the project's standards.
I'd appreciate the opportunity to work on it and can start right away

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

[ibochielizabeth-spec]

@ibochielizabeth-spec has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

hello maintainer i would like to fix this issues quicky....

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

[drips-wave]

Congratulations, @ibochielizabeth-spec! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @ibochielizabeth-spec: 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.

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