Architecture: complete DataKeyCore/Scoped/Ext storage schema

[josephchimebuka]

Summary

Finish the XDR-safe key split. Remove ad-hoc Symbol keys where typed keys belong; eliminate mixed legacy DataKey usage.

Acceptance criteria

• Documented key taxonomy
• No dual-write ambiguity
• Migration notes if key paths change

Where

• contracts/src/types.rs
• contracts/src/storage.rs
• contracts/src/config.rs

[AGWAM001]

@AGWAM001 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I'd love to take this issue on. I'm a full-stack developer with experience across Drips Waves4-7.
My stack include rust, Soroban, Solidity, and TypeScript. I can deliver a clean, tested, and well-documented solution within 24 hours of assignment.
Jovial to start immediately

ℹ️ Repo Maintainers: To accept this application, review their application or assign @AGWAM001 to this issue.