feat: add pytest unit tests and CI workflow (#38)

* feat: add pytest unit tests and CI workflow

Add 67 tests covering both create_listing.py and dedup.py with shared
Pillow-based image fixtures. Add GitHub Actions workflow to run tests
on Python file changes.

* fix: address PR review feedback

- Use monkeypatch.chdir(tmp_path) so tests write to temp dirs instead
  of polluting the repo's README.md and index.html
- Strengthen unicode filename test to assert URL-encoded form (%C3%A9)
- Move hashlib import to module level in test_dedup.py
- Remove unused _zero_hash helper and Path import
- Prefix unused tuple unpacking variables with underscore

* fix: add docstrings and strengthen degenerate hash test

- Add docstrings to all test classes, methods, and helper functions
  to achieve 100% docstring coverage
- Strengthen test_skips_degenerate_hashes to assert groups == []
  instead of only checking for no-crash

* fix: use hardcoded MD5 digests and add fixture validation

- Replace hashlib.md5() calls with known digest constants to remove
  hashlib import from test module
- Add input validation to _make_gif fixture for clear error messages
  on empty colors or mismatched durations length

tests/conftest.py

@@ -0,0 +1,89 @@
+"""Shared test fixtures for emoji project tests."""
+
+from pathlib import Path
+
+import pytest
+from PIL import Image
+
+
+@pytest.fixture
+def make_png():
+    """Factory fixture: creates a small PNG image and returns its Path."""
+
+    def _make_png(
+        directory: Path,
+        name: str,
+        color: tuple = (255, 0, 0, 255),
+        size: tuple[int, int] = (4, 4),
+    ) -> Path:
+        img = Image.new("RGBA", size, color)
+        path = directory / name
+        img.save(path, "PNG")
+        return path
+
+    return _make_png
+
+
+@pytest.fixture
+def make_gif():
+    """Factory fixture: creates an animated GIF with multiple frames and returns its Path."""
+
+    def _make_gif(
+        directory: Path,
+        name: str,
+        colors: list[tuple],
+        durations: list[int],
+        size: tuple[int, int] = (4, 4),
+    ) -> Path:
+        if not colors:
+            raise ValueError("colors must not be empty")
+        if len(durations) != len(colors):
+            raise ValueError(f"durations length ({len(durations)}) must match colors length ({len(colors)})")
+        frames = [Image.new("RGBA", size, c) for c in colors]
+        path = directory / name
+        frames[0].save(
+            path,
+            save_all=True,
+            append_images=frames[1:],
+            duration=durations,
+            loop=0,
+        )
+        return path
+
+    return _make_gif
+
+
+@pytest.fixture
+def make_jpg():
+    """Factory fixture: creates a small JPEG image and returns its Path."""
+
+    def _make_jpg(
+        directory: Path,
+        name: str,
+        color: tuple = (255, 0, 0),
+        size: tuple[int, int] = (4, 4),
+    ) -> Path:
+        img = Image.new("RGB", size, color)
+        path = directory / name
+        img.save(path, "JPEG")
+        return path
+
+    return _make_jpg
+
+
+@pytest.fixture
+def emoji_dir(tmp_path, make_png, make_gif, make_jpg):
+    """Creates a temp directory with several named test images."""
+    d = tmp_path / "emoji"
+    d.mkdir()
+    make_png(d, "alpha.png", color=(255, 0, 0, 255))
+    make_png(d, "beta.png", color=(0, 255, 0, 255))
+    make_png(d, "gamma.png", color=(0, 0, 255, 255))
+    make_jpg(d, "delta.jpg", color=(128, 128, 0))
+    make_gif(
+        d,
+        "animated.gif",
+        colors=[(255, 0, 0, 255), (0, 255, 0, 255)],
+        durations=[100, 100],
+    )
+    return d