Migration annotations

Add one YAML file per v3 module. Each file maps stable API ids (without the snapshot's trailing #type or #value facet) to migration guidance:

effect/Effect#async:
  replacement: Effect.callback
  note: Use the callback constructor.
  example: Effect.callback((resume) => resume(Effect.void))

Run pnpm api-diff --check to list missing ids and pnpm api-diff --write-doc migration/v3-to-v4.md to regenerate the reference.