feat: Add metadata namespace and fix stats calculation

This is a major release with breaking changes to metadata format.

BREAKING CHANGES:
- All metadata keys now use 'dg-' namespace prefix (becomes 'x-amz-meta-dg-*' in S3)
- Old metadata format is not supported - all files must be re-uploaded
- Stats behavior changed: quick mode no longer shows misleading warnings

Features:
- Metadata now uses real package version (dg-tool: deltaglider/VERSION)
- All metadata keys properly namespaced with 'dg-' prefix
- Clean stats output in quick mode (no per-file warning spam)
- Fixed nonsensical negative compression ratios in quick mode

Fixes:
- Stats now correctly handles delta files without metadata
- Space saved shows 0 instead of negative numbers when metadata unavailable
- Removed misleading warnings in quick mode (metadata not fetched is expected)
- Fixed metadata keys to use hyphens instead of underscores

Documentation:
- Added comprehensive metadata documentation
- Added stats calculation behavior guide
- Added real version tracking documentation

Tests:
- Updated all tests to use new dg- prefixed metadata keys
- All 73 unit tests passing
- All quality checks passing (ruff, mypy)

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

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

src/deltaglider/client.py

@@ -9,6 +9,7 @@ from collections.abc import Callable
 from pathlib import Path
 from typing import Any, cast
 
+from . import __version__
 from .adapters.storage_s3 import S3StorageAdapter
 from .client_delete_helpers import delete_with_delta_suffix
 from .client_models import (
@@ -1325,8 +1326,8 @@ def create_client(
     logger = StdLoggerAdapter(level=log_level)
     metrics = NoopMetricsAdapter()
 
-    # Get default values
-    tool_version = kwargs.pop("tool_version", "deltaglider/5.0.0")
+    # Get default values (use real package version)
+    tool_version = kwargs.pop("tool_version", f"deltaglider/{__version__}")
     max_ratio = kwargs.pop("max_ratio", 0.5)
 
     # Create service