`overlay` behavior inconsistent with Postgres / SQL spec

### Describe the bug

```
  SELECT overlay('abc' placing 'X' from 4 for 1);
  -- DF returns: 'X'
  -- Postgres returns: 'abcX'
```

```
  SELECT overlay('abc' placing 'X' from 0 for 1);
  -- DF returns: 'Xabc'
  -- Postgres produces "ERROR: negative substring length not allowed"
```

### To Reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_