StringifyMapper: Incorrect result for side-by-side product nodes

```python
>>> import pymbolic.primitives as p
>>> a = p.Variable("a")
>>> b = p.Variable("b")
>>> c = p.Variable("c")
>>> print(a * (b * c))
a*b*c # Should have been 'a*(b*c)'
```

Start by locating the StringifyMapper entry point responsible for rendering nested product nodes and reproduce the issue with the provided Python example. Check how parenthesized products are handled, then add a regression test showing that a * (b * c) stringifies as a*(b*c) rather than a*b*c.

Written by the indexing model from the issue text.