[Contract] Implement On-Chain Audit Logging Module

[yusuftomilola]

An audit trail module that records significant events for each asset on-chain, accessible to any caller.

Requirements

AuditEntry struct (in audit.rs): timestamp: u64, action: String, actor: Address, details: String

append_audit_log(env, asset_id, action, actor, details) — Internal function. Reads the current log vec for the asset, appends the new entry, stores back. Used by asset, tokenization, insurance, and leasing modules to record events.

get_asset_log(env, asset_id) — Returns Vec<AuditEntry> (empty vec if no history).

Integration: Wire append_audit_log calls into register_asset, transfer_asset_ownership, retire_asset, and major state changes in tokenization/insurance/leasing.

Acceptance Criteria

• Audit log grows correctly with each recorded action
• Log retrieval returns entries in chronological order
• Internal function cannot be called directly from outside the contract