Scene exits, Back(), and the current/previous scene accessors

A connection between two scenes had to be hand-built as a hotspot: an area,
a cursor, a GoTo, and a flag check written out again for every door. Scene
now carries Exits, and the engine expands each one into a hotspot.

- Exit{To, Label, Side, Area, Needs, Blocked, OnLook, OnTake} in scene.exit.go.
  With no Area an exit is an edge strip, sized as a fraction of Game.SceneArea()
  so a game whose HUD covers the foot of the window gets strips inside the
  painting. Game.ExitLook / Game.ExitTake supply the look and take responses
  once for the whole game, so the phrasing is not repeated per exit.
- Game.SceneHotspots(name) is the scene's own hotspots followed by its exits,
  cached per scene. Authored hotspots come first, so a painted thing beats the
  edge strip wherever the two overlap. HotspotAt and HotspotDebug read it.
- Game.CurrentScene() / PreviousScene(), both persisted in the save file. The
  engine tracked the current scene but exposed no accessor, so every game had
  to shadow it to know where it was.
- Back() returns to the previous scene — what an exit with an empty To binds
  to, for a scene reachable from several rooms whose way out is a direction
  rather than a place.
- Validate rejects an exit naming a scene nobody registered.

Generated hotspots are named ExitName(To) ("exit:street", "exit:back"), so the
location graph can be read straight back out of the registered scenes.

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

action.def.go

@@ -250,6 +250,24 @@ func (a *gotoAction) Tick(ctx *Ctx) Status {
 	return StatusDone
 }
 
+// ----- Back -------------------------------------------------------------
+
+type backAction struct{}
+
+// Back returns to the scene the player came from. Most exits name their
+// destination, because a door leads where it leads; a scene reached from
+// several different rooms cannot, so its way out is a direction, not a place.
+// A no-op when there is nowhere to go back to.
+func Back() Action { return backAction{} }
+
+func (a backAction) Start() Runner { return a }
+func (a backAction) Tick(ctx *Ctx) Status {
+	if prev := ctx.Game.PreviousScene(); prev != "" {
+		ctx.Game.changeScene(prev)
+	}
+	return StatusDone
+}
+
 // ----- inventory --------------------------------------------------------
 
 type giveAction struct{ item string }