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>

scene.exit.go

@@ -0,0 +1,121 @@
+package inkwell
+
+// ExitSide is where on the picture an exit sits when it has no Area of its
+// own: off one edge, into the depth of the picture, or out towards the
+// camera. Until the doors are measured on the artwork, an edge strip is the
+// convention every 1990s point & click used, and re-aiming an exit at a real
+// door later is a one-field change.
+type ExitSide int
+
+const (
+	ExitLeft  ExitSide = iota // off the left edge
+	ExitRight                 // off the right edge
+	ExitBack                  // into the depth of the picture
+	ExitNear                  // out towards the camera
+)
+
+// Exit is one way out of a scene. The engine turns it into a Hotspot, so a
+// connection can be declared as data — where it leads, what the status line
+// calls it, and what has to be true before it opens — instead of being spelled
+// out as another hand-built hotspot.
+//
+// A connection stays machine-readable: the generated hotspot is named
+// ExitName(To), so the location graph can be read straight back out of the
+// registered scenes.
+type Exit struct {
+	// To is the scene the exit leads to. Empty means "back the way you came" —
+	// for a scene reachable from several others, where the way out is a
+	// direction rather than a place.
+	To string
+
+	// Label is what the status line calls it.
+	Label string
+
+	// Side places the exit when Area is nil.
+	Side ExitSide
+
+	// Area overrides the edge strip Side would give it.
+	Area Shape
+
+	// Needs is a flag that has to be set before the exit opens; Blocked is
+	// what happens while it is not. The exit is visible either way — a locked
+	// door still says it is a door.
+	Needs   string
+	Blocked Action
+
+	// OnLook and OnTake override Game.ExitLook and Game.ExitTake for this one
+	// exit.
+	OnLook Action
+	OnTake Action
+}
+
+// ExitName is the hotspot name generated for an exit to the named scene.
+func ExitName(to string) string {
+	if to == "" {
+		return "exit:back"
+	}
+	return "exit:" + to
+}
+
+func (e Exit) hotspot(g *Game) Hotspot {
+	var travel Action = GoTo(e.To)
+	if e.To == "" {
+		travel = Back()
+	}
+	if e.Needs != "" {
+		if e.Blocked != nil {
+			travel = If(Flag(e.Needs), travel, e.Blocked)
+		} else {
+			travel = If(Flag(e.Needs), travel)
+		}
+	}
+	area := e.Area
+	if area == nil {
+		area = e.Side.area(g.SceneArea())
+	}
+	look, take := e.OnLook, e.OnTake
+	if look == nil && g.ExitLook != nil {
+		look = g.ExitLook(e)
+	}
+	if take == nil && g.ExitTake != nil {
+		take = g.ExitTake(e)
+	}
+	return Hotspot{
+		Name:   ExitName(e.To),
+		Label:  e.Label,
+		Area:   area,
+		Cursor: CursorExit,
+		OnLook: look,
+		OnUse:  travel,
+		OnTake: take,
+	}
+}
+
+// The edge strips, as fractions of the scene area, so a game that hands over
+// only part of the window to the picture (a HUD along the foot, letterbox
+// bars) gets strips that sit inside the picture rather than under the HUD.
+const (
+	exitEdgeWidth  = 0.0875
+	exitEdgeInset  = 0.080
+	exitEdgeHeight = 0.876
+	exitDoorWidth  = 0.275
+	exitBackInset  = 0.033
+	exitBackHeight = 0.347
+	exitNearHeight = 0.198
+)
+
+func (s ExitSide) area(r Rectangle) Shape {
+	edge := r.W * exitEdgeWidth
+	door := r.W * exitDoorWidth
+	doorX := r.X + (r.W-door)/2
+	switch s {
+	case ExitLeft:
+		return Rect(r.X, r.Y+r.H*exitEdgeInset, edge, r.H*exitEdgeHeight)
+	case ExitRight:
+		return Rect(r.X+r.W-edge, r.Y+r.H*exitEdgeInset, edge, r.H*exitEdgeHeight)
+	case ExitBack:
+		return Rect(doorX, r.Y+r.H*exitBackInset, door, r.H*exitBackHeight)
+	default:
+		return Rect(doorX, r.Y+r.H*(1-exitNearHeight), door, r.H*exitNearHeight)
+	}
+}