feat: Implement Content-Addressed Storage (CAS) cache

Implemented SHA256-based Content-Addressed Storage to eliminate
cache collisions and enable automatic deduplication.

Key Features:
- Zero collision risk: SHA256 namespace guarantees uniqueness
- Automatic deduplication: same content = same filename
- Tampering protection: changing content changes SHA, breaks lookup
- Two-level directory structure (ab/cd/abcdef...) for filesystem optimization

Changes:
- Added ContentAddressedCache adapter in adapters/cache_cas.py
- Updated CLI and SDK to use CAS instead of FsCacheAdapter
- Updated all tests to use ContentAddressedCache
- Documented CAS architecture in CLAUDE.md and SECURITY_FIX_ROADMAP.md

Security Benefits:
- Eliminates cross-endpoint collision vulnerabilities
- Self-describing cache (filename IS the checksum)
- Natural cache validation without external metadata

All quality checks passing:
- 99 tests passing (0 failures)
- Type checking: 0 errors (mypy)
- Linting: All checks passed (ruff)

Completed Phase 2 of SECURITY_FIX_ROADMAP.md

src/deltaglider/client.py

@@ -1114,7 +1114,7 @@ def create_client(
     """
     # Import here to avoid circular dependency
     from .adapters import (
-        FsCacheAdapter,
+        ContentAddressedCache,
         NoopMetricsAdapter,
         S3StorageAdapter,
         Sha256Adapter,
@@ -1143,7 +1143,10 @@ def create_client(
     hasher = Sha256Adapter()
     storage = S3StorageAdapter(endpoint_url=endpoint_url, boto3_kwargs=boto3_kwargs)
     diff = XdeltaAdapter()
-    cache = FsCacheAdapter(cache_dir, hasher)
+
+    # SECURITY: Use Content-Addressed Storage for zero-collision guarantee
+    cache = ContentAddressedCache(cache_dir, hasher)
+
     clock = UtcClockAdapter()
     logger = StdLoggerAdapter(level=log_level)
     metrics = NoopMetricsAdapter()