Function exec_id_at 

pub fn exec_id_at(unix: Duration) -> String

A fresh exec id for the wall time unix, since the epoch: x- plus 16 hex characters, the Python client’s shape.

The id needs to be distinct rather than unguessable — it is an idempotency key, not a credential, and the daemon rejects an unknown one — so the nanosecond clock mixed with a counter is enough. Code holding a crate::session::Session mints through the session’s clock, so a test that pins the clock pins the id. microvms_core::session::mint_exec_id is this on the production clock.

Reads no clock: the time is the caller’s. A process-wide counter still separates two ids minted for the same instant.