feat(security): audit cross-contract calls and implement reentrancy guard (closes #354)

[shobhamerabacha-star]

Summary

Closes #354

Delivers a comprehensive cross-contract security audit and robust reentrancy protection architecture across the ChainLearn smart contract ecosystem (learn-token, credential-nft, and progress-tracker).

Key Changes

• Cross-Contract Audit Report (docs/reentrancy-audit.md):
  • Full inventory of external invocations (invoke_contract calls) in learn-token (claim_reward, batch_claim_reward, preview_claim_reward) and credential-nft (mint_credential).
  • Threat modeling and vulnerability identification (same-function reentrancy during score verification, cross-function reentrancy into token operations, and credential minting recursion).
  • Remediation architecture and formal verification notes.
• RAII Reentrancy Guard Architecture:
  • contracts/learn-token/src/storage.rs: Added TokenDataKey::ReentrancyGuard in instance storage with is_reentrancy_locked and set_reentrancy_locked.
  • contracts/learn-token/src/lib.rs: Introduced ReentrancyGuard RAII struct enforcing zero reentrant entries across claim_reward, batch_claim_reward, mint, transfer, transfer_from, burn, burn_from, and claim_vested.
  • contracts/credential-nft/src/metadata.rs & contracts/credential-nft/src/lib.rs: Added CredentialDataKey::ReentrancyGuard and protected mint_credential during external progress-tracker verification.
• Unit & Integration Attack Suite (tests/integration/security_reentrancy_tests.rs):
  • test_reentrancy_during_claim_reward_blocked_by_guard: Adversarial progress tracker attempts recursive claim_reward during quiz score resolution; halted with panic, 0 tokens minted.
  • test_cross_function_reentrancy_blocked_by_guard: Adversarial tracker attempts mint during score resolution; halted, total supply and balances remain uncorrupted.
  • test_credential_nft_reentrancy_blocked_by_guard: Adversarial tracker attempts nested mint_credential during eligibility verification; blocked, credential counter remains 0.
  • test_reentrancy_prevented_state_consistent_and_no_funds_lost: Invariance verification under failed reentrancy attacks.
  • test_reentrancy_during_transfer: Transfer level reentrancy prevention.
• Contract Tests:
  • cargo test -p learn-token: 78/78 tests passed.
  • cargo test -p credential-nft: 37/37 tests passed.
  • cargo test --test security_reentrancy_tests: 5/5 tests passed.

Verification

• cargo test -p learn-token: 78 passed, 0 failed.
• cargo test -p credential-nft: 37 passed, 0 failed.
• cargo test --test security_reentrancy_tests: 5 passed, 0 failed.