//! Tests for the inbox substrate (ARCH ยง2.11), split by axis so each
//! file stays under the repo's per-file line cap.
//!
//! - [`lock`]: `flock` acquire / exclusion / release-on-drop and the
//! errno interpretation.
//! - [`deposit`]: create-only atomicity, frontmatter, and sender
//! sequence derivation.
//! - [`probe`]: the deposit-starts-a-driver decision, sender
//! resolution, and the `cli_message` orchestration.