@spec preflight(String.t(), String.t()) ::
  {:ok, map()} | {:error, Backplane.AgentRuntime.Error.t()}

Inspects all ordered targets using the same parser as apply/2, without effects.

The digest binds the inspection to the exact raw patch. Paths reflect current filesystem containment and write safety; no file contents are read, and missing update/delete sources are allowed. Execution remains sequential and may be partial. A later filesystem change requires fresh authorization or OS isolation.