Sync engine refactor

The engine fetches, diffs and writes in one pass. Split it into three stages so each one can be tested on its own.

Stages

Rollout

• Freeze the current engine API
• Port fetch and diff to the new stages
• −Switch all traffic to the new engine
• +Run parity tests on a 5% shadow sync
• +Keep a rollback switch for one release

Sketch

export async function sync(repo: Repo) {
  const remote = await fetchStage(repo)
  const delta  = diffStage(repo.local, remote)
  await writeStage(repo, delta)  // one stage, one test
}