Tesla investors have a lot to parse after the July 4 holiday: a disappointing quarter of deliveries, a record month of production, and now several weeks of downtime at multiple plants. The electric-car maker will halt most production on its Model Y assembly line in Shanghai for the first two weeks of July, then stop the Model 3 line for a 20-day stretch starting July 18, Bloomberg reported last month. Upgrade work at the factory to boost the…