In-memory LangEx.Checkpointer backend.

Checkpoints live in a public ETS table owned by the application — dependency-free and fast, but per-VM and lost on restart. Use for development, tests, and short-lived graphs; production threads that must survive restarts belong in LangEx.Checkpointer.Redis or LangEx.Checkpointer.Postgres.

graph = Graph.compile(builder, checkpointer: LangEx.Checkpointer.Memory)