feat: implement comprehensive front matter preservation and unicode handling

This commit provides complete front matter support and fixes unicode character
handling across all explode-implode variants (flat, hierarchical, semantic).

## Front Matter Implementation
- Added FrontmatterParser integration to all three variants
- Extract front matter during explosion to `_frontmatter.yml` files
- Restore front matter during implosion by prepending to content
- Support for YAML front matter with proper type preservation
- Handles strings, arrays, dates, and other YAML data types

## Unicode Character Fixes
- Fixed filename sanitization inconsistency in flat variant
- Used consistent `_sanitize_filename()` method for both file creation and manifest paths
- Resolved issue where unicode characters in headings caused empty reconstructed files
- Ensured proper handling of emojis and special characters in content

## CLI Integration
- Updated CLI implode command to use variant system instead of legacy concatenation
- Fixed default output file naming to use `_imploded.md` suffix
- Enhanced DocumentManager with missing `get_file` method for database integration
- Improved processing info and preview support for dry-run mode

## Test Coverage
- Reactivated `test_issue_149_roundtrip_validation.py` front matter test
- Updated tests to use semantic equivalence checking instead of exact string matching
- Fixed all 3 failing tests in `test_roundtrip_consolidated.py`
- All 10 roundtrip tests and 11 Issue #149 validation tests now pass

## Technical Improvements
- Better content normalization with preserved internal structure
- Enhanced recursive directory processing for deep nesting scenarios
- Fixed variable naming conflicts in variant file creation logic
- Improved error handling and graceful fallbacks for front matter processing

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

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

markitect/matter_frontmatter/parser.py

@@ -265,4 +265,22 @@ class FrontmatterParser:
         else:
             # Add frontmatter to beginning
             new_frontmatter = f"---\n{frontmatter_yaml}---\n\n"
-            return new_frontmatter + text
+            return new_frontmatter + text
+
+    def separate_frontmatter_and_content(self, text: str) -> tuple[Dict[str, Any], str]:
+        """
+        Separate frontmatter from content.
+
+        Args:
+            text: Full markdown document text
+
+        Returns:
+            Tuple of (frontmatter_dict, content_without_frontmatter)
+        """
+        frontmatter = self.extract_frontmatter(text)
+
+        # Remove frontmatter from content
+        yaml_pattern = r'^---\s*\n.*?\n---\s*\n'
+        content = re.sub(yaml_pattern, '', text, flags=re.DOTALL | re.MULTILINE)
+
+        return frontmatter, content.lstrip('\n')