Wire backend capacity into engine runtime

types.go

@@ -561,10 +561,13 @@ type StructuredOutputJSONSpec struct {
 
 // LLMClient executes rendered prompts for [Engine.Run].
 //
-// Generate may be called concurrently. It must honor context cancellation to
-// make Run responsive to cancellation. The request and all nested maps,
-// slices, and pointers are client-owned copies and may be mutated or retained
-// without affecting engine state.
+// Generate is scheduled according to the resolved backend's capacity policy.
+// It may still be called concurrently for different backend pools or unlimited
+// backends. Cancellation while waiting for capacity can prevent Generate from
+// being called. Once invoked, it must honor context cancellation to make Run
+// responsive to cancellation. The request and all nested maps, slices, and
+// pointers are client-owned copies and may be mutated or retained without
+// affecting engine state.
 //
 // Generate receives rendered messages and may receive a direct API key. A
 // client must protect those values and any raw output in its logging, storage,