Add pacman-speed-ms constant for automatic movement interval

Adds a 200ms movement tick rate, controlling how fast Pac-Man moves
automatically through the maze.

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

pacman-project/constants.rkt

@@ -33,6 +33,9 @@
           sprite-scale-key
           sprite-scale-key-ui
 
+          ;; Movement
+          pacman-speed-ms
+
           ;; Animation
           animation-interval-ms
 
@@ -93,6 +96,9 @@
     (define sprite-scale-key 1.5)
     (define sprite-scale-key-ui 3)
 
+    ;; Movement speed: time between automatic movement ticks
+    (define pacman-speed-ms 200)
+
     ;; Animation timing
     (define animation-interval-ms 100)