docs: test fixtures README and ADR-006 cleanup

Rakefile

@@ -35,8 +35,18 @@ task "report:sample", [:embed] do |_t, args|
   ruby "scripts/generate_sample_report.rb #{embed_arg}"
 end
 
+desc "Remove screenshot diff artifacts (keeps baselines)"
+task "snap_diff:clean" do
+  patterns = ["**/*.diff.png", "**/*.base.diff.png", "**/*.heatmap.diff.png",
+    "**/*.diff.webp", "**/*.base.diff.webp", "**/*.heatmap.diff.webp",
+    "**/snap_diff_report.html"]
+  removed = patterns.flat_map { |p| Dir.glob("tmp/#{p}") + Dir.glob("doc/screenshots/#{p}") }
+  removed.each { |f| FileUtils.rm_f(f) }
+  puts "Removed #{removed.size} diff artifacts"
+end
+
 task "clobber" do
-  puts "Cleanup tmp/*.png"
+  puts "Cleanup tmp/"
   FileUtils.rm_rf(Dir["./tmp/*"])
 end
 

test/fixtures/images/README.md

@@ -0,0 +1,24 @@
+# Test Fixture Images
+
+These images are used by the unit test suite for image comparison testing.
+
+| File | Description |
+|------|-------------|
+| `a.png` | Base island map (80x80) |
+| `b.png` | Modified island map — small region differs from `a.png` |
+| `c.png` | Modified island map — different region differs from `a.png` |
+| `d.png` | Modified island map — another variation |
+| `a_cropped.png` | Cropped version of `a.png` |
+| `portrait.png` | Portrait orientation image (3x6) |
+| `portrait_b.png` | Modified portrait — differs from `portrait.png` |
+| `a.webp` | WebP version of `a.png` |
+
+## Generated artifacts (gitignored)
+
+These files are generated during test runs and should NOT be committed:
+
+| Pattern | Description |
+|---------|-------------|
+| `*.diff.png` | Annotated diff overlay |
+| `*.base.diff.png` | Annotated base image |
+| `*.heatmap.diff.png` | Heatmap of pixel differences |