Initial commit: DeltaGlider - 99.9% compression for S3 storage

DeltaGlider reduces storage costs by storing only binary deltas between
similar files. Achieves 99.9% compression for versioned artifacts.

Key features:
- Intelligent file type detection (delta for archives, direct for others)
- Drop-in S3 replacement with automatic compression
- SHA256 integrity verification on every operation
- Clean hexagonal architecture
- Full test coverage
- Production tested with 200K+ files

Case study: ReadOnlyREST reduced 4TB to 5GB (99.9% compression)

tests/conftest.py

@@ -0,0 +1,101 @@
+"""Pytest configuration and fixtures."""
+
+import shutil
+import tempfile
+from pathlib import Path
+from unittest.mock import Mock
+
+import pytest
+
+from deltaglider.adapters import (
+    FsCacheAdapter,
+    NoopMetricsAdapter,
+    Sha256Adapter,
+    StdLoggerAdapter,
+    UtcClockAdapter,
+)
+from deltaglider.core import DeltaService
+
+
+@pytest.fixture
+def temp_dir():
+    """Create temporary directory."""
+    with tempfile.TemporaryDirectory() as tmpdir:
+        yield Path(tmpdir)
+
+
+@pytest.fixture
+def sample_file(temp_dir):
+    """Create sample test file."""
+    file_path = temp_dir / "test.zip"
+    file_path.write_text("Sample content for testing")
+    return file_path
+
+
+@pytest.fixture
+def mock_storage():
+    """Create mock storage port."""
+    return Mock()
+
+
+@pytest.fixture
+def mock_diff():
+    """Create mock diff port."""
+    mock = Mock()
+    # Make encode create empty delta file
+    def encode_side_effect(base, target, out):
+        out.write_bytes(b"delta content")
+    mock.encode.side_effect = encode_side_effect
+    return mock
+
+
+@pytest.fixture
+def real_hasher():
+    """Create real SHA256 hasher."""
+    return Sha256Adapter()
+
+
+@pytest.fixture
+def cache_adapter(temp_dir, real_hasher):
+    """Create filesystem cache adapter."""
+    cache_dir = temp_dir / "cache"
+    return FsCacheAdapter(cache_dir, real_hasher)
+
+
+@pytest.fixture
+def clock_adapter():
+    """Create UTC clock adapter."""
+    return UtcClockAdapter()
+
+
+@pytest.fixture
+def logger_adapter():
+    """Create logger adapter."""
+    return StdLoggerAdapter(level="DEBUG")
+
+
+@pytest.fixture
+def metrics_adapter():
+    """Create metrics adapter."""
+    return NoopMetricsAdapter()
+
+
+@pytest.fixture
+def service(mock_storage, mock_diff, real_hasher, cache_adapter, clock_adapter, logger_adapter, metrics_adapter):
+    """Create DeltaService with test adapters."""
+    return DeltaService(
+        storage=mock_storage,
+        diff=mock_diff,
+        hasher=real_hasher,
+        cache=cache_adapter,
+        clock=clock_adapter,
+        logger=logger_adapter,
+        metrics=metrics_adapter,
+    )
+
+
+@pytest.fixture
+def skip_if_no_xdelta():
+    """Skip test if xdelta3 not available."""
+    if shutil.which("xdelta3") is None:
+        pytest.skip("xdelta3 not available")