Fix(Colors): Use standard Racket color names and round coin dots

The (pp1 graphics) library resolves colors via Racket's color database,
which doesn't support hex strings — they return #f causing a contract
violation on set-brush.

Replaced all hex colors with standard names:
  #2121DE -> "medium blue", #FFB851 -> "gold", #FFB8FF -> "hot pink",
  #FFFF00 -> "yellow", #FF0000 -> "red", #111111 -> "dark slate gray"

Also switched coins from draw-rectangle! to draw-ellipse! for round
dot rendering (arcade-accurate).

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

pacman-project/adt/draw.rkt

@@ -154,13 +154,13 @@
                  color-door))))))
 
         ;; draw-coins! :: maze -> /
-        ;; Redraws all coins as small centered dots.
+        ;; Redraws all coins as small round dots.
         (define (draw-coins! maze)
           ((coins-tile 'clear!))
           ((maze 'for-each-cell)
            (lambda (row col cell-type)
              (when (= cell-type cell-type-coin)
-               ((coins-tile 'draw-rectangle!)
+               ((coins-tile 'draw-ellipse!)
                 (+ (grid->pixel-x col) coin-inset)
                 (+ (grid->pixel-y row) coin-inset)
                 coin-size

pacman-project/constants.rkt

@@ -147,17 +147,17 @@
     (define rotation-up 90)
     (define rotation-down -90)
 
-    ;; Colors — arcade-style palette
+    ;; Colors — arcade-style palette (standard Racket color names only)
     (define color-background "black")
-    (define color-wall "#2121DE")
+    (define color-wall "medium blue")
-    (define color-door "#FFB8FF")
+    (define color-door "hot pink")
-    (define color-coin "#FFB851")
+    (define color-coin "gold")
     (define color-text "white")
-    (define color-title "#FFFF00")
+    (define color-title "yellow")
-    (define color-header-bg "#111111")
+    (define color-header-bg "dark slate gray")
-    (define color-game-over "#FF0000")
+    (define color-game-over "red")
     (define color-pause-bg "black")
-    (define color-pause-text "#FF0000")
+    (define color-pause-text "red")
 
     ;; UI layout — header bar at the top
     (define header-height 90)