download: free the -j slot once a torrent starts seeding

download/download.go

@@ -64,3 +64,14 @@ type Download interface {
 	Run(ctx context.Context) error
 	Stat() Stat
 }
+
+// Seeder is an optional capability of a Download whose work continues after its
+// content is fully downloaded — a torrent that keeps running to seed.
+// SeedingStarted is closed at that download->seed transition. The engine uses it
+// to free the concurrency slot the moment a download stops downloading: a
+// seed-only torrent is no longer a download, so it must not count against
+// --max-concurrent-downloads (-j) and block a queued download. HTTP downloads do
+// not implement this, so they hold their slot until they finish.
+type Seeder interface {
+	SeedingStarted() <-chan struct{}
+}