Audit trail with tamper evident

[gloskull]

Motivation
Provide a compact, on-chain tamper-evident audit trail so off-chain monitors can detect deletion, reordering, or mutation of critical lifecycle and billing events.
Cover high-value operations (meter registration, funding/top-up, and signed usage deductions) so settlement and monitoring systems can verify integrity end-to-end.
Description
Add a new AuditRecord contract type that stores sequence, previous_hash, record_hash, action, subject_id, actor, timestamp, and payload_hash for compact, privacy-preserving verification.
Add storage keys DataKey::AuditHead and DataKey::AuditRecord(u64) and implement helpers zero_hash, audit_payload_hash, calculate_audit_record_hash, and append_audit_record to maintain a chained hash head and persist per-sequence records.
Append audit records from critical paths by calling the append helper in register_meter, top_up, and deduct_units, and emit an AuditTrail event when records are appended.
Expose monitor-facing contract methods get_audit_head, get_audit_record, and verify_audit_chain to allow off-chain watchers to fetch and verify inclusive ranges of the chain, and add AUDIT_TRAIL_RUNBOOK.md with verification, alerting, and deployment guidance.
Testing
Ran cargo test --manifest-path contracts/utility_contracts/Cargo.toml audit --lib, which did not complete successfully because the workspace currently has pre-existing unrelated compile errors (e.g. duplicate MIGRATION_INSTRUCTION_BUDGET and outdated test/client signatures).
Ran cargo fmt --manifest-path contracts/utility_contracts/Cargo.toml --check, which failed because several repository files are not rustfmt-clean; no broad formatting changes were applied to avoid unrelated churn.
No new unit tests were added for the audit helpers in this change set; monitors can exercise the new public helpers via integration testing and canary deployments as described in the runbook.
Closes #92