Contracts docs: `proposals` storage layout & treasury interface

[codebestia]

Write contracts/docs/contracts-proposals-storage.md documenting contracts/contracts/proposals/src/storage.rs, treasury_interface.rs, and treasury_interface_client.rs — the Proposal/ProposalStatus data structures, vote-tracking storage, and the typed cross-contract client used to call into group_treasury.

Acceptance criteria:

• Every storage key/data structure is documented with its type, including the ProposalStatus enum's full set of values and transitions
• The treasury_interface_client.rs cross-contract-call mechanism is documented — how a contract ID is resolved and invoked
• Storage type and TTL/bump behavior is documented

[JesseJohn7]

@JesseJohn7 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Please assign me to this issue

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[Kingvic300]

@Kingvic300 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would love to work on this kindly assign it to me

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[Purity-Euphemia]

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

Please I can work on this 🙏

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[drips-wave]

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

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