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/explode_variants/enums.py

@@ -62,6 +62,34 @@ class ExplodeVariant(Enum):
     └── appendices/
     """
 
+    MDZ = "mdz"
+    """
+    Packaging variant for creating compressed packages (.mdz format).
+    Creates self-contained packages with embedded assets and metadata.
+
+    Example:
+    document.mdz (ZIP archive containing):
+    ├── content.md
+    ├── manifest.json
+    └── assets/
+        ├── image1.png
+        └── style.css
+    """
+
+    MDT = "mdt"
+    """
+    Packaging variant for creating template packages (.mdt format).
+    Creates template packages with variable substitution and conditional content.
+
+    Example:
+    template.mdt (archive containing):
+    ├── template.md
+    ├── variables.json
+    └── assets/
+        ├── template.css
+        └── default.png
+    """
+
 
 class ExplodeMode(Enum):
     """

markitect/explode_variants/variant_factory.py

@@ -15,6 +15,33 @@ from .hierarchical_variant import HierarchicalVariant
 from .semantic_variant import SemanticVariant
 from .variant_detector import VariantDetector, DetectionResult
 
+# Packaging variants are imported lazily to avoid circular imports
+_MDZ_AVAILABLE = None  # Lazy evaluation
+_MDZ_IMPORT_ERROR = None
+_MdzVariant = None  # Cached import
+
+
+def _check_mdz_availability():
+    """Check if MDZ variant is available, with lazy import."""
+    global _MDZ_AVAILABLE, _MDZ_IMPORT_ERROR, _MdzVariant
+
+    if _MDZ_AVAILABLE is not None:
+        return _MDZ_AVAILABLE
+
+    try:
+        from ..packaging.mdz_variant import MdzVariant
+        _MdzVariant = MdzVariant
+        _MDZ_AVAILABLE = True
+        return True
+    except ImportError as e:
+        _MDZ_AVAILABLE = False
+        _MDZ_IMPORT_ERROR = str(e)
+        return False
+    except Exception as e:
+        _MDZ_AVAILABLE = False
+        _MDZ_IMPORT_ERROR = f"Unexpected error: {e}"
+        return False
+
 
 class VariantFactory:
     """
@@ -39,6 +66,10 @@ class VariantFactory:
         self.register_variant(ExplodeVariant.HIERARCHICAL, HierarchicalVariant)
         self.register_variant(ExplodeVariant.SEMANTIC, SemanticVariant)
 
+        # Register packaging variants if available (lazy loading)
+        if _check_mdz_availability():
+            self.register_variant(ExplodeVariant.MDZ, _MdzVariant)
+
     def register_variant(self, variant_type: ExplodeVariant, variant_class: Type[BaseVariant]) -> None:
         """
         Register a variant class with the factory.