Implement data migration/import-export (#46) and advanced security/audit trail (#47)

[Jagadeeshftw]

Summary

Implements #46 Data Migration and Import/Export Utilities and #47 Advanced Security Features and Audit Trail, and ensures CI passes (build, test, clippy, fmt).

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#46 – Data Migration and Import/Export

New crate: data_migration

• Export formats: JSON (human-readable), binary (bincode), CSV (goals), base64 “encrypted” payload (caller encrypts/decrypts).
• Checksum: SHA256 of payload; validated on import.
• Version: SCHEMA_VERSION and MIN_SUPPORTED_VERSION; version compatibility checked on import.
• API: export_to_json, export_to_binary, export_to_csv, export_to_encrypted_payload, import_from_json, import_from_binary, import_goals_from_csv, import_from_encrypted_payload, validate_for_import, check_version_compatibility.
• Rollback: RollbackMetadata for migration scripts.

Contract changes

• remittance_split: export_snapshot(env, caller), import_snapshot(env, caller, nonce, snapshot) with version and checksum validation (owner-only).
• savings_goals: export_snapshot(env, caller), import_snapshot(env, caller, nonce, snapshot) for full goals backup/restore with validation.

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#47 – Advanced Security and Audit Trail

remittance_split

• Audit trail: AuditEntry (operation, caller, timestamp, success). All state-changing ops log success/failure; get_audit_log(env, from_index, limit) (capped at 100 entries).
• Nonce & replay protection: get_nonce(env, address); state-changing functions require correct nonce: initialize_split, update_split, distribute_usdc.
• Integer overflow: calculate_split uses checked_mul / checked_div / checked_sub.

savings_goals

• Audit trail: Same pattern for create_goal, add_to_goal, withdraw_from_goal, lock_goal, unlock_goal; get_audit_log(env, from_index, limit).
• Nonce: Used for import_snapshot replay protection; get_nonce(env, address).
• Integer overflow: add_to_goal uses checked_add, withdraw_from_goal uses checked_sub.

(Workspace already has overflow-checks = true in release.)

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Breaking API changes

• remittance_split:
  • initialize_split(env, owner, nonce, spending_percent, ...)
  • update_split(env, caller, nonce, ...)
  • distribute_usdc(env, usdc_contract, from, nonce, accounts, total_amount)
• Callers must pass the current nonce (from get_nonce) for these functions.

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Testing & CI

• New tests: data_migration (checksum, JSON/binary/CSV export-import, version checks), remittance_split export_import_snapshot_and_audit and replay_attack_rejected.
• Existing tests updated for new nonce parameters.
• CI: cargo build --release --target wasm32-unknown-unknown, cargo test --all-features, cargo clippy --all-targets --all-features -- -D warnings, cargo fmt --all -- --check all pass.

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Closes #46
Closes #47

[Baskarayelu]

Looks good to me @Jagadeeshftw