test: add CSSCache unit tests and benchmarks

Tests cover:
- get/0, put/1, invalidate/0, warm/0 functions
- Performance benchmark comparing cache hit vs CSS generation
- 100 cache hit consistency test

Run with: mix test test/simpleshop_theme/theme/css_cache_test.exs --include benchmark

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test/simpleshop_theme/theme/css_cache_test.exs

@@ -0,0 +1,109 @@
+defmodule SimpleshopTheme.Theme.CSSCacheTest do
+  use SimpleshopTheme.DataCase
+
+  alias SimpleshopTheme.Theme.CSSCache
+  alias SimpleshopTheme.Theme.CSSGenerator
+  alias SimpleshopTheme.Settings.ThemeSettings
+
+  describe "get/0" do
+    test "returns cached CSS after warm" do
+      # Cache should be pre-warmed on startup
+      assert {:ok, css} = CSSCache.get()
+      assert is_binary(css)
+      assert css =~ ".themed {"
+    end
+  end
+
+  describe "put/1 and get/0" do
+    test "stores and retrieves CSS" do
+      test_css = "/* test css */"
+      :ok = CSSCache.put(test_css)
+
+      assert {:ok, ^test_css} = CSSCache.get()
+
+      # Restore original cache
+      CSSCache.warm()
+    end
+  end
+
+  describe "invalidate/0" do
+    test "clears cached CSS" do
+      # Ensure cache has something
+      CSSCache.warm()
+      assert {:ok, _} = CSSCache.get()
+
+      # Invalidate
+      :ok = CSSCache.invalidate()
+      assert :miss = CSSCache.get()
+
+      # Restore
+      CSSCache.warm()
+    end
+  end
+
+  describe "warm/0" do
+    test "populates cache from current settings" do
+      CSSCache.invalidate()
+      assert :miss = CSSCache.get()
+
+      :ok = CSSCache.warm()
+      assert {:ok, css} = CSSCache.get()
+      assert is_binary(css)
+    end
+  end
+
+  describe "performance" do
+    @tag :benchmark
+    test "cache hit is faster than generation" do
+      # Ensure cache is warm
+      CSSCache.warm()
+      settings = %ThemeSettings{}
+
+      # Benchmark cache hit (should be microseconds)
+      {cache_time, {:ok, _cached_css}} =
+        :timer.tc(fn ->
+          CSSCache.get()
+        end)
+
+      # Benchmark CSS generation (should be milliseconds)
+      {gen_time, _generated_css} =
+        :timer.tc(fn ->
+          CSSGenerator.generate(settings)
+        end)
+
+      # Log results for visibility
+      IO.puts("\n")
+      IO.puts("  Cache hit:     #{cache_time} µs")
+      IO.puts("  CSS generate:  #{gen_time} µs")
+      IO.puts("  Speedup:       #{Float.round(gen_time / max(cache_time, 1), 1)}x faster")
+
+      # Cache should be faster than generation
+      assert cache_time < gen_time,
+             "Cache (#{cache_time}µs) should be faster than generation (#{gen_time}µs)"
+    end
+
+    @tag :benchmark
+    test "multiple cache hits are consistent" do
+      CSSCache.warm()
+
+      times =
+        for _ <- 1..100 do
+          {time, {:ok, _}} = :timer.tc(fn -> CSSCache.get() end)
+          time
+        end
+
+      avg = Enum.sum(times) / length(times)
+      max_time = Enum.max(times)
+      min_time = Enum.min(times)
+
+      IO.puts("\n")
+      IO.puts("  100 cache hits:")
+      IO.puts("    Avg: #{Float.round(avg, 1)} µs")
+      IO.puts("    Min: #{min_time} µs")
+      IO.puts("    Max: #{max_time} µs")
+
+      # Average should be under 100 microseconds (ETS is fast)
+      assert avg < 100, "Average cache hit (#{avg}µs) should be under 100µs"
+    end
+  end
+end