Drop the duplicate Manager, use inkwell's

The engine already had the registry the game was reimplementing:
inkwell.Manager[T Named], and every entity type here is an alias of an
engine struct, so they all carry GetName() already. manager.manager.go
and manager.interface.go are gone; a category manager is now one call to
inkwell.NewManager.

registerAll goes with it — Each does the hand-off — and the selector
writes its derived pins back with Set, which keeps the scene in place in
the registration order.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

go.mod

@@ -3,7 +3,7 @@ module git.teletypegames.org/games/realworld
 go 1.26.3
 
 require (
-	git.teletypegames.org/engines/inkwell v0.1.1-0.20260829220920-f9745e426624
+	git.teletypegames.org/engines/inkwell v0.1.1-0.20260830075028-08f15a7e3d11
 	github.com/hajimehoshi/ebiten/v2 v2.9.9
 )