Support registering global traits at client initialization

## Summary
Some traits are global and never change for the life of a client, for example app name and developer branch. Today they must be supplied on every evaluation call. Consumers work around this by wrapping the client to register global traits and merge them with per-call traits at runtime.

## Requested feature
A way to register global traits once at client initialization so they are included in every evaluation by default, removing the per-call merge or custom wrapper.

Start by locating the Python client initialization and evaluation methods, then trace how per-call traits are currently passed. Define an initialization option for global traits and verify that evaluations include them by default while still accepting per-call traits. Done means consumers no longer need a wrapper or manual merge for traits that remain constant.

Rédigé par le modèle d'indexation à partir du texte de l'issue.