Use anonymous namespaces for private functions in C++ files

Turns out just because a function is only defined in a `.cpp` file, doesn't mean that it is private. E.g., I added
```c++
// flow.hpp
int namespace_experiment(int a) {
return a;
}
```
and then
```c++
// test_flow.cpp
int namespace_experiment(int a);
```
and that works just fine despite the declaration not being in the corresponding header, but rather an unrelated one.

But adding
```c++
// flow.hpp
namespace {
int namespace_experiment(int a) {
return a;
}
}
```
correctly makes it private.

We should fix this everywhere.

#### AI Generation Disclosure
I found this out when Claude used an anonymous namespace in https://github.com/dwavesystems/dwave-optimization/pull/590 which prompted me to learn more about it.

Start by inspecting the C++ source files for functions defined only in .cpp files, using flow.hpp and test_flow.cpp as the example of the visibility issue. Identify file-local functions that lack anonymous namespaces and update the affected files; the work is done when private functions are consistently restricted across the project and the existing tests still pass.

Written by the indexing model from the issue text.