`Spring.GetUnitShieldState` returns bool as number

```
Spring.GetUnitShieldState(unitID, weaponNum?) -> number isEnabled, number shieldHealth
^^^^^^
should be bool instead of 0/1
```

The caveat is backward compatibility.

Start by locating the implementation and API declaration for Spring.GetUnitShieldState, then inspect how its return values are exposed to callers. Determine how the requested bool return can be handled with the stated backward-compatibility caveat; done means the behavior and compatibility impact are clearly resolved.

Written by the indexing model from the issue text.