Docs/consolidate contracts md

[TheBigWealth89]

Consolidate smart contract documentation into CONTRACTS.md

Resolves: #137

Description

This PR consolidates fragmented smart contract documentation across the repository into a single, canonical CONTRACTS.md file, acting as the ultimate source of truth for on-chain logic.

Scope of Changes

• Created CONTRACTS.md as the unified technical specification and overview.
• Merged and removed 15 fragmented documentation files (e.g., SETTLEMENT_IMPLEMENTATION.md, mint-atomicity.md, macro-auth.md, SECURITY.md, etc.).
• Extracted and embedded current ground-truth public interfaces and structs from .rs source code.
• Left backend, infrastructure, and off-chain operational runbooks untouched (e.g. WEBHOOK_ARCHITECTURE.md, EMERGENCY_RUNBOOK.md).
• Replaced the root README.md with a minimalist pointer to CONTRACTS.md.

Review Notes / Open Questions

• Exclusions: Several .md files in docs/ and root (e.g. CONTRIBUTING.md, backend runbooks) were explicitly kept out of the merge as they do not document the on-chain contract logic. Please confirm this separation is correct.

Verification

• cargo test passes successfully.