docs(campaign): add campaign contract architecture system design doc

[devNuel]

Overview

This PR adds a Campaign Contract Architecture & System Design document that captures the campaign contract's state machine, security model, and scalability design, and links it from the README so contributors and auditors have a single source of truth for the contract's expected behavior and evolution.

Related Issue

Closes the issue: docs(campaign): Campaign contract architecture - system design doc

Changes

📐 Campaign Contract Architecture Design Doc

• [ADD] CAMPAIGN_CONTRACT_ARCHITECTURE.md

  • Documents the campaign lifecycle state machine (Draft, Active, Paused, Canceled, Expired, Settled) with transition rules, guards, and invariants.
  • Details the security model: roles and permissions, owner/operator actions, escrow and payment safety, reentrancy safeguards, emergency pause/refund paths, and key threat mitigations.
  • Describes the scalability design: on-chain vs. off-chain data strategy, pagination and read patterns, batching campaign operations, event indexing, and growth limits.

• [MODIFY] README.md

  • Adds an Architecture section with a brief summary of the campaign contract design doc and a direct link to CAMPAIGN_CONTRACT_ARCHITECTURE.md.

Verification Results

npx markdownlint README.md CAMPAIGN_CONTRACT_ARCHITECTURE.md
✅ 2/2 files passed

Docs acceptance check:
✅ State machine covers all campaign states and transitions
✅ Security model includes roles, threats, and mitigations
✅ Scalability section documents storage, batching, and read patterns
✅ README link to architecture doc resolves

Acceptance Criteria	Status
Campaign contract state machine is documented	✅ All states, transition rules, and invariants defined
Security model is documented	✅ Roles, permissions, escrow, and threat mitigations covered
Scalability design is documented	✅ Storage, batching, pagination, and indexing patterns described
README links to the architecture doc	✅ Link added in README and verified in docs check

Closes #744

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[coderabbitai]

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[Utilitycoder]

Thanks for the work. Kindly ensure you offramp with Fundable at https://stellar.fundable.finance/offramp