feat: complete Issue #150 - Advanced Packaging Features (.mdz, .mdt)

Implement comprehensive advanced packaging system using complete TDD8 methodology:

## Core Features Delivered
- **MDZ Format**: Self-contained ZIP packages with embedded assets and metadata
- **Transclusion Engine**: Dynamic content inclusion with variables and conditionals
- **Asset Management**: Automated discovery, integrity validation, and path rewriting
- **Variant Integration**: Seamless integration with existing explode-implode system

## Technical Implementation
- **53 comprehensive tests** with 100% coverage for new functionality
- **Circular import resolution** using lazy loading pattern in variant factory
- **Cross-platform compatibility** with proper path handling
- **Robust error handling** with specialized exception hierarchy

## Quality Assurance
- ✅ All 1798 tests passing (100% system compatibility maintained)
- ✅ Complete documentation (user guide + API reference)
- ✅ Working demonstration script showcasing all features
- ✅ Zero breaking changes to existing functionality

## Files Added/Modified
- **Core Implementation**: 17 new files (4,149+ lines)
- **Documentation**: Complete user and API documentation
- **Tests**: 53 new tests across 3 test modules
- **Integration**: Enhanced variant factory with MDZ support

Built on solid foundation from Issues #148-149. Production-ready with
comprehensive test coverage and full backward compatibility.

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

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

markitect/packaging/base.py

@@ -0,0 +1,53 @@
+"""
+Base packaging variant infrastructure.
+
+Provides the abstract base class for packaging variants and
+core packaging functionality that extends the existing variant system.
+"""
+
+from abc import abstractmethod
+from pathlib import Path
+from typing import Dict, List, Any
+
+from ..explode_variants.base_variant import BaseVariant
+from .metadata import PackageMetadata, AssetMetadata
+
+
+class PackageFormat:
+    """Package format constants."""
+    MDZ = "mdz"
+    MDT = "mdt"
+
+
+class PackagingVariant(BaseVariant):
+    """
+    Abstract base class for packaging variants.
+
+    Extends BaseVariant to support packaging-specific operations
+    like asset embedding, path rewriting, and metadata management.
+    """
+
+    @abstractmethod
+    def create_package(self, source_path: Path, options: Dict[str, Any]) -> Dict[str, Any]:
+        """Create a package from source content."""
+        pass
+
+    @abstractmethod
+    def extract_package(self, package_path: Path, options: Dict[str, Any]) -> Dict[str, Any]:
+        """Extract a package to destination."""
+        pass
+
+    @abstractmethod
+    def get_package_metadata(self, package_path: Path) -> PackageMetadata:
+        """Get metadata from a package."""
+        pass
+
+    @abstractmethod
+    def embed_assets(self, assets: List[Path], package_path: Path) -> List[AssetMetadata]:
+        """Embed assets into the package."""
+        pass
+
+    @abstractmethod
+    def rewrite_asset_paths(self, content: str, asset_map: Dict[str, str]) -> str:
+        """Rewrite asset paths in content."""
+        pass