Preflight the exact private archive before expensive GPU work

A code-only environment preflight cannot validate modules that exist only inside a later private archive. Before expensive GPU work, extract the exact sealed archive that will execute and run its real first import path using the target interpreter and dependency environment. Put this gate before model downloads, dataset preprocessing, and optimizer creation, and record it as a durable stage. Keep code-only preflight for image and dependency compatibility; it complements, but does not replace, artifact-level execution smoke tests.

Applies when: private training or inference payloads are uploaded after a public runtime preflight. Keywords: GPU preflight private archive exact artifact import smoke fail closed