Optimize(Level): Add change notification callbacks for draw invalidation

Level now fires callbacks when game state changes that require redrawing:
- on-coins-changed!: fired when a coin is eaten or key is picked up
- on-maze-changed!: fired when a door is removed

Exposes set-on-coins-changed! and set-on-maze-changed! messages so the
game ADT can wire these to the draw ADT's dirty flags.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pacman-project/adt/level.rkt

@@ -33,7 +33,9 @@
             (timer (make-timer))
             (paused? #f)
             (queued-direction #f)
-            (movement-timer 0))
+            (movement-timer 0)
+            (on-coins-changed! (lambda () #f))
+            (on-maze-changed! (lambda () #f)))
 
         ;; Initialize key after maze is created.
         (set! key (make-key maze))
@@ -77,7 +79,8 @@
         (define (eat-coin! row col)
           ((maze 'cell-set!) row col cell-type-empty)
           ((score 'increase!))
-          ((timer 'increase!)))
+          ((timer 'increase!))
+          (on-coins-changed!))
 
         ;;
         ;; Key logic
@@ -87,7 +90,8 @@
         ;; Picks up the key and clears the cell.
         (define (pick-up-key! row col)
           ((maze 'cell-set!) row col cell-type-empty)
-          ((key 'take!)))
+          ((key 'take!))
+          (on-coins-changed!))
 
         ;;
         ;; Teleportation logic
@@ -130,7 +134,8 @@
               ;; Door: open it if key has been taken.
               (((maze 'door?) next-row next-col)
                (when (key 'taken?)
-                 ((maze 'remove-door!) next-row next-col)))
+                 ((maze 'remove-door!) next-row next-col)
+                   (on-maze-changed!)))
 
               ;; Normal movement: only if not a wall.
               (else
@@ -201,6 +206,16 @@
         ;; Dispatch
         ;;
 
+        ;; set-on-coins-changed! :: (-> /) -> /
+        ;; Registers a callback for when coins change (eaten or key picked up).
+        (define (set-on-coins-changed! callback)
+          (set! on-coins-changed! callback))
+
+        ;; set-on-maze-changed! :: (-> /) -> /
+        ;; Registers a callback for when the maze changes (door removed).
+        (define (set-on-maze-changed! callback)
+          (set! on-maze-changed! callback))
+
         (define (dispatch-level msg)
           (cond ((eq? msg 'maze) maze)
                 ((eq? msg 'pacman) pacman)
@@ -210,6 +225,8 @@
                 ((eq? msg 'paused?) paused?)
                 ((eq? msg 'key-press!) key-press!)
                 ((eq? msg 'update!) update!)
+                ((eq? msg 'set-on-coins-changed!) set-on-coins-changed!)
+                ((eq? msg 'set-on-maze-changed!) set-on-maze-changed!)
                 (else (error "Level ADT -- Unknown message:" msg))))
 
         dispatch-level))))