Universal formattable concept

### Describe the need of your project

There's a zoo of formattable concepts in AUI:
- `operator<<` (default c++)
- `fmt::formattable` (from {fmt})
- `PrintTo` (from gtest)

They are not interchangeable. I want implement formattable/printable concept once per type instead implementing it 3x times per type.

### Proposed solution

Take `fmt::formattable` as base formatting concept. As AUI uses both GTest and {fmt}, implement GTest's `PrintTo` function and use `fmt::formattable` there.

### Alternatives you've considered

_No response_

### Additional context

_No response_

Start by locating AUI's existing operator<< implementations and its integrations with fmt::formattable and GTest PrintTo; compare how each is used. Done means one per-type formatting concept can serve both {fmt} and GTest without requiring three implementations, with behavior checked across both consumers.

Written by the indexing model from the issue text.