Route every launch through the front door so routing can be decided there #397

A launch only reaches the door when a provider, headroom or usage tracking is on. A plain launch talks straight to Anthropic with a base URL fixed at launch, so nothing downstream can add headroom to it later. Engaging the door for every launch (except --native) makes the per-session decision possible.

It also makes every session depend on the door, and a door turnover already strands sessions on stale ports, so this follows the turnover fixes: a superseded door stops taking sessions (#371), a failed probe no longer spawns a second door (#370), and refused launches after a turnover are resolved (#367).