deepcopy(nav) causes RecursionError as navigation grows

Both _filter_topic_nav and _filter_section_nav use deepcopy(nav) to create a mutable copy of the navigation before filtering it. The MkDocs Navigation object has circular references (Page.ancestors → Section → Section.children → Page → ...), so deepcopy recursively traverses these until it hits Python's recursion limit. The issue doesn't surface on small sites but triggers once the nav is big enough

How to reproduce - Any mkdocs project using topic-nav with enough pages in the navigation. Adding pages to an existing site that was previously just under the limit will suddenly break the build.

I came across this when adding a page to diamond's dev-guide:

```
...
File "/usr/local/lib/python3.9/dist-packages/mkdocs/plugins.py", line 507, in run_event
result = method(item, **kwargs)
File "/usr/local/lib/python3.9/dist-packages/mkdocs_topic_nav/plugin.py", line 33, in on_page_context
context["nav"] = self._filter_topic_nav(nav, page)
File "/usr/local/lib/python3.9/dist-packages/mkdocs_topic_nav/plugin.py", line 63, in _filter_topic_nav
new_nav = deepcopy(nav)
File "/usr/lib/python3.9/copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
File "/usr/lib/python3.9/copy.py", line 270, in _reconstruct
state = deepcopy(state, memo)
File "/usr/lib/python3.9/copy.py", line 146, in deepcopy
y = copier(x, memo)
File "/usr/lib/python3.9/copy.py", line 230, in _deepcopy_dict
y[deepcopy(key, memo)] = deepcopy(value, memo)
File "/usr/lib/python3.9/copy.py", line 146, in deepcopy
y = copier(x, memo)
File "/usr/lib/python3.9/copy.py", line 205, in _deepcopy_list
append(deepcopy(a, memo))
File "/usr/lib/python3.9/copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
...
```

Start in mkdocs_topic_nav/plugin.py at _filter_topic_nav and _filter_section_nav, where the issue identifies deepcopy(nav) as the failing operation. Reproduce the build with a sufficiently large navigation, then verify that topic and section filtering complete without RecursionError while preserving the filtered navigation behavior.

Written by the indexing model from the issue text.