Concepts docs: Proposal lifecycle & voting model

[codebestia]

Write contracts/docs/concepts-proposal-lifecycle.md explaining the proposals contract's voting design: how a proposal is created, how votes are weighted/counted, what finalization does, expiry handling, and how a finalized proposal's execution ties back into group_treasury withdrawals.

Acceptance criteria:

• A state diagram covers every ProposalStatus value and the function calls that cause each transition
• Explains what determines a proposal passing or failing (vote count/threshold/quorum — whichever the code actually implements) precisely
• Explains the expiry path and why finalize_expired_proposal exists as distinct from normal finalization

[jaymoneymanxl]

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

I would love to work on this, Please assign it to me

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

[Ajibola6921]

@Ajibola6921 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello Maintainer, I'd be glad to take responsibility for this issue and submit a high-quality pull request for your review.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Ajibola6921 to this issue.

[drips-wave]

Congratulations, @Ajibola6921! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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