Function update_checkpoint_cache 

pub fn update_checkpoint_cache(
    cache_path: &Path,
    new_checkpoint: &SignedCheckpoint,
    consistency_proof: &ConsistencyProof,
    _trust_root: &TrustRoot,
    _now: DateTime<Utc>,
) -> Result<ConsistencyReport, TransparencyWorkflowError>

Update the local checkpoint cache after verifying consistency.

Loads the cached checkpoint from disk, verifies that the new checkpoint is a consistent append-only extension of the cached one, and writes the new checkpoint to disk.

Note: Uses blocking std::fs I/O (not tokio::fs). This is acceptable for the current use case — a single small JSON file read/write from CLI context. If called from a multi-threaded async server, wrap in tokio::task::spawn_blocking.

Args:

• cache_path — Path to the cached checkpoint JSON file.
• new_checkpoint — The newly received signed checkpoint.
• consistency_proof — Proof that old tree is a prefix of the new tree.
• _trust_root — Trust root for checkpoint signature verification (reserved for future use).
• _now — Injected wall-clock time (reserved for future use).

Usage:

let report = update_checkpoint_cache(
    &cache_path,
    &new_checkpoint,
    &consistency_proof,
    &trust_root,
    now,
)?;