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>

tests/test_roundtrip_consolidated.py

@@ -267,11 +267,19 @@ End of document.
             ])
             assert result.returncode == 0
 
-            # Verify front matter preservation
+            # Verify front matter preservation - check for semantic equivalence
             reconstructed_content = reconstructed_file.read_text(encoding='utf-8')
-            assert 'title: "Test Document"' in reconstructed_content
-            assert 'author: "Test Author"' in reconstructed_content
-            assert "tags:" in reconstructed_content
+
+            # Use frontmatter parser to check semantic equivalence
+            from markitect.matter_frontmatter.parser import FrontmatterParser
+            parser = FrontmatterParser()
+            reconstructed_fm = parser.extract_frontmatter(reconstructed_content)
+
+            # Check that all expected values are preserved
+            assert reconstructed_fm.get('title') == 'Test Document'
+            assert reconstructed_fm.get('author') == 'Test Author'
+            assert reconstructed_fm.get('tags') == ['test', 'markdown']
+            assert reconstructed_fm.get('version') == 1.0
 
     def test_unicode_and_special_characters_roundtrip(self):
         """Test roundtrip with unicode and special characters."""