Function init 

pub fn init(
    path: &Path,
    redact_content: bool,
    append_only_hint: bool,
) -> Result<()>

Initialise the audit sink. Called at most once per process from init_from_config; subsequent calls replace the prior sink so test-only callers can swap targets.

Errors

• The audit directory cannot be created.
• The audit log file cannot be opened in append mode.
• Reading the existing chain tail (to seed last_hash) fails.