Variables `Final` by default (`mutable` / `var`)

```py
a = 1
a = 2 # should be an error imo
```
```py
a: var = 1
a = 2 # yeah, that's ok
```

Use the two Python examples as the behavioral specification: inspect the assignment and annotation/type-checking entry points, then find the existing tests for reassignment. Done means unannotated variables reject reassignment by default while `var` permits it, with tests covering both cases.

Written by the indexing model from the issue text.