feat: implement PBKDF2 algorithm with registry

tests/test_algorithms.py

@@ -21,3 +21,20 @@ def test_get_algorithm_unknown_raises_error():
     """Verify unknown algorithm raises ValueError."""
     with pytest.raises(ValueError, match="Unknown algorithm"):
         get_algorithm("unknown")
+
+
+def test_pbkdf2_algorithm_hash_round_trip():
+    """Verify PBKDF2 algorithm can hash and verify passwords."""
+    algo = get_algorithm("pbkdf2")
+    salt, hashed = algo.hash("test password")
+    assert algo.verify("test password", salt, hashed)
+    assert not algo.verify("wrong password", salt, hashed)
+
+
+def test_pbkdf2_algorithm_respects_iterations():
+    """Verify PBKDF2 algorithm respects custom iterations."""
+    algo = get_algorithm("pbkdf2")
+    salt1, hash1 = algo.hash("test", iterations=100000)
+    salt2, hash2 = algo.hash("test", iterations=200000)
+    # Different iterations should produce different hashes even with same password
+    assert hash1 != hash2