fix: improve FlatVariant bridge method and add consolidated roundtrip tests

🔧 Fixes:
- Fix FlatVariant bridge method to properly create temp files for implode operations
- Resolve placeholder content issue in roundtrip tests
- Exclude manifest.md from processed files list

🧪 Testing:
- Add comprehensive consolidated roundtrip test suite
- Test all variants with CLI integration
- Include error handling and edge case testing

📊 Status:
- Legacy roundtrip tests: 10/11 passing (1 architectural difference)
- Variant system core functionality: Working
- CLI integration: Minor issues to resolve

Files Added:
- tests/test_roundtrip_consolidated.py

Files Modified:
- markitect/explode_variants/flat_variant.py

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

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

markitect/explode_variants/flat_variant.py

@@ -300,29 +300,42 @@ class FlatVariant(BaseVariant):
         try:
             from markitect.plugins.builtin.markdown_commands import cli_implode_directory
 
-            # Use existing implode logic
+            # Create a temporary file for the existing implode logic
+            import tempfile
+            with tempfile.NamedTemporaryFile(mode='w+', suffix='.md', delete=False) as temp_file:
+                temp_path = Path(temp_file.name)
+
+            # Use existing implode logic with actual file creation
             result = cli_implode_directory(
                 input_dir=input_directory,
-                output_file=options.output_file or Path("/tmp/temp.md"),
-                dry_run=True,  # We handle file writing ourselves
+                output_file=temp_path,
+                dry_run=False,  # Actually create the file so we can read it
                 verbose=options.verbose,
-                overwrite=options.overwrite,
+                overwrite=True,  # Always overwrite temp file
                 preserve_front_matter=options.preserve_front_matter,
                 section_spacing=options.section_spacing
             )
 
-            if result.success:
-                # Read the content that would have been written
-                temp_file = options.output_file or Path("/tmp/temp.md")
-                if temp_file.exists():
-                    content = temp_file.read_text(encoding='utf-8')
-                else:
-                    content = "# Imploded Content\n\n(Content generation in progress...)"
+            if result.success and temp_path.exists():
+                # Read the generated content
+                content = temp_path.read_text(encoding='utf-8')
+                # Exclude manifest from processed files
+                files_processed = [f for f in input_directory.glob("**/*.md") if f.name != "manifest.md"]
+
+                # Clean up temp file
+                try:
+                    temp_path.unlink()
+                except Exception:
+                    pass
 
-                files_processed = list(input_directory.glob("**/*.md"))
                 return content, files_processed
             else:
-                raise Exception(result.error_message or "Implosion failed")
+                # Clean up temp file
+                try:
+                    temp_path.unlink()
+                except Exception:
+                    pass
+                raise Exception(result.error_message if hasattr(result, 'error_message') else "Implosion failed")
 
         except ImportError:
             # Fallback basic implementation for testing