feat: add algorithm interface and registry

tests/test_algorithms.py

@@ -0,0 +1,23 @@
+import pytest
+
+from algorithms import Algorithm, get_algorithm
+
+
+def test_algorithm_has_required_methods():
+    """Verify Algorithm protocol defines required methods."""
+    algo = get_algorithm("pbkdf2")
+    assert hasattr(algo, "hash")
+    assert hasattr(algo, "verify")
+    assert hasattr(algo, "identifier")
+
+
+def test_get_algorithm_returns_pbkdf2():
+    """Verify default algorithm is PBKDF2."""
+    algo = get_algorithm("pbkdf2")
+    assert algo.identifier == "pbkdf2"
+
+
+def test_get_algorithm_unknown_raises_error():
+    """Verify unknown algorithm raises ValueError."""
+    with pytest.raises(ValueError, match="Unknown algorithm"):
+        get_algorithm("unknown")