"Trace one image all the way around: Photoshop pixel, to brain, to engine, to candidate, and back. Every arrow in that loop is a seam — and the seams are the architecture."
The Whole Loop, Traced Once
Now the three parts move together. Follow a single refinement from start to finish: the artist selects a region in Photoshop; the thin bridge captures it and sends it to the brain; the brain stamps lineage and forwards a generate request to the engine; the engine runs the adapter, the modules, the sampler, and returns image bytes; the brain records the candidate and pushes it to the workspace; the artist picks one, and it goes back through the bridge into Photoshop as a non-destructive layer. Every track you've done is one segment of this loop.
The Brain Is the Single Fan-In
Notice what the loop does not do: the workspace never talks to the engine directly. Every generation request fans in through the brain. The brain is the one door into the engine. The workspace asks the brain; the bridge asks the brain; any future client asks the brain. None of them open their own connection to the engine. That single fan-in is a deliberate architectural choice, not an accident of wiring.
What the Single Door Buys
Concentrating access at the brain pays off three ways. Authentication lives in one place — the engine trusts the brain, and the brain vets everyone else. Lineage is stamped consistently, because every request passes the one point that records it. Routing decisions (which adapter, which model) happen once, in the brain, rather than being re-derived by each client. Three cross-cutting concerns, one place to get them right.
Why Not Let the Workspace Call the Engine Directly?
It would be faster to wire the workspace straight to the engine — one less hop. But then the workspace would need its own copy of auth, its own lineage stamping, its own routing logic, and so would the next client, and the one after. Each copy is a place for the implementations to diverge and the rules to drift. The extra hop through the brain is the price of having those rules in exactly one place. A little latency buys a lot of consistency.