Stub color extractor in tests

Makes the test suite much faster

test/support/fake_color_extractor.ex

@@ -0,0 +1,14 @@
+defmodule MusicLibrary.Colors.FakeColorExtractor do
+  @moduledoc """
+  Test stub for `MusicLibrary.Colors.KMeansExtractor`.
+
+  Returns hardcoded colors to avoid CPU-intensive K-means clustering in tests.
+  """
+
+  @behaviour MusicLibrary.Colors.Extractor
+
+  @impl true
+  def extract_dominant_colors(_image_data, _num_colors \\ 5) do
+    {:ok, ["#000000", "#c0c0c0", "#c08080", "#404000", "#804040"]}
+  end
+end