Exarch’s modules form a DAG

Exarch’s top-level modules form a directed acyclic graph: a crate:: reference from a module to one that already reaches it, outside tests and comments, is a build break.

The layers, leaves first (see exarch-architecture and exarch):

• clock, latch, cancel, card, app, skill, library
• provider
• record
• bus
• schedule
• enquiry
• shell_eval
• agent
• boot
• tui, headless
• cli, lib

Each boundary says what the lower layer is spared:

• provider knows no record: a provider speaks the wire and nothing of the session’s durable log.
• record knows no bus: it publishes through its Publish trait, never to a concrete bus.
• bus knows no agent: the inbox’s Stamp and cancel::Token suffice.
• shell_eval knows no agent: the desk is reached through ral_core::carrier::Host.
• prompt and enquiry know no agent: both are read by the agent, never the converse.

Pinned by exarch/tests/layering.rs.