Per-session journal for the Chio runtime.

This crate provides an append-only, hash-chained journal that tracks request history, cumulative data flow (bytes read/written), delegation depth, and tool invocation sequence within a single session.

The journal persists across requests within a session and is available to all guards. Entries are tamper-evident: each entry includes a SHA-256 hash of the previous entry, forming a hash chain.

Design

• Append-only: entries can only be added, never modified or removed.
• Hash-chained: each entry hashes the previous entry's hash for tamper detection.
• Thread-safe: the journal is wrapped in a Mutex for safe concurrent access from multiple guards.