Choose the sync engine we will invest in next

## What happens now

The project has three sync engines, but we have not chosen which one should receive most of our engineering attention after September.

## What should happen instead

Choose one candidate using agreed criteria and record the evidence, risks, and reasons for the decision.

This is a stub and still needs a real example, completion checks, and implementation notes.

Start by reviewing the project’s three sync engine candidates and define agreed criteria for comparing them. Add a real example, completion checks, implementation notes, and the evidence, risks, and reasons behind the selected engine.

Written by the indexing model from the issue text.