bm_ao_render.rb: use pack() for binary PPM output

Replace printf("%c", val) with pack("CCC") for outputting raw
bytes. printf("%c") with values >= 128 produces invalid UTF-8
on CRuby, corrupting the PPM output. Also remove unused
arguments from the "255\n" format string.

Co-authored-by: Claude <noreply@anthropic.com>

benchmark/bm_ao_render.rb

@@ -292,9 +292,7 @@ class Scene
         r = rad.x / nsfs
         g = rad.y / nsfs
         b = rad.z / nsfs
-        printf("%c", clamp(r))
-        printf("%c", clamp(g))
-        printf("%c", clamp(b))
+        print([clamp(r), clamp(g), clamp(b)].pack("CCC"))
       end
     end
   end
@@ -303,7 +301,7 @@ end
 # File.open("ao.ppm", "w") do |fp|
   printf("P6\n")
   printf("%d %d\n", IMAGE_WIDTH, IMAGE_HEIGHT)
-  printf("255\n", IMAGE_WIDTH, IMAGE_HEIGHT)
+  printf("255\n")
   Scene.new.render(IMAGE_WIDTH, IMAGE_HEIGHT, NSUBSAMPLES)
 #  Scene.new.render(256, 256, 2)
 # end