refactor: Rename Leaf to DeltaSpace for semantic clarity

- Renamed Leaf class to DeltaSpace throughout the codebase
- Updated all imports, method signatures, and variable names
- Updated documentation and comments to reflect the new naming
- DeltaSpace better represents a container for delta-compressed files

The term "DeltaSpace" is more semantically accurate than "Leaf" as it
represents a space/container for managing related files with delta
compression, not a terminal node in a tree structure.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/deltaglider/adapters/cache_fs.py

@@ -15,30 +15,30 @@ class FsCacheAdapter(CachePort):
         self.base_dir = base_dir
         self.hasher = hasher
 
-    def ref_path(self, bucket: str, leaf: str) -> Path:
+    def ref_path(self, bucket: str, prefix: str) -> Path:
         """Get path where reference should be cached."""
-        cache_dir = self.base_dir / bucket / leaf
+        cache_dir = self.base_dir / bucket / prefix
         return cache_dir / "reference.bin"
 
-    def has_ref(self, bucket: str, leaf: str, sha: str) -> bool:
+    def has_ref(self, bucket: str, prefix: str, sha: str) -> bool:
         """Check if reference exists and matches SHA."""
-        path = self.ref_path(bucket, leaf)
+        path = self.ref_path(bucket, prefix)
         if not path.exists():
             return False
 
         actual_sha = self.hasher.sha256(path)
         return actual_sha == sha
 
-    def write_ref(self, bucket: str, leaf: str, src: Path) -> Path:
+    def write_ref(self, bucket: str, prefix: str, src: Path) -> Path:
         """Cache reference file."""
-        path = self.ref_path(bucket, leaf)
+        path = self.ref_path(bucket, prefix)
         path.parent.mkdir(parents=True, exist_ok=True)
         shutil.copy2(src, path)
         return path
 
-    def evict(self, bucket: str, leaf: str) -> None:
+    def evict(self, bucket: str, prefix: str) -> None:
         """Remove cached reference."""
-        path = self.ref_path(bucket, leaf)
+        path = self.ref_path(bucket, prefix)
         if path.exists():
             path.unlink()
         # Clean up empty directories