Durable stepper and storage adapters for Statifier.

This module itself carries only version/0. The package's surface is four modules, and the README's worked example drives all of them in order:

• StatifierPersistence.Storage - the identity-guarded facade over charts, positions, and run records. Every load is guarded against the chart revision that wrote it; there is no unguarded path.
• StatifierPersistence.Storage.Adapter - the behaviour a backing store implements. StatifierPersistence.Storage.InMemory is the reference implementation, StatifierPersistence.Storage.Ecto the Postgres one, and StatifierPersistence.Testing.StorageConformance holds any third to the same bar.
• StatifierPersistence.Runs - the lifecycle: create/4, step/5, fail/4, each running load -> step -> execute effects -> persist in ADR-0004 decision 3's fixed order, with no live Session process.
• StatifierPersistence.Executor - the seam every effect crosses on its way to the host, as a module or a bare arity-2 fun.

StatifierPersistence.Serialization is the fifth, and usually invisible: it orders concurrent deliveries to one run, defaulting to the adapter's own lock_run/3.