Add manifest generation to include_assets build step

- Add generateManifest boolean to IncludeAssetsConfigSchema (default false)
- Add anchor and groupBy fields to ConfigKeyEntrySchema for grouped manifest entries
- Track copy operations in pathMap to resolve output-relative paths
- Deduplicate source paths to prevent indexed suffixes on shared files
- Use sequential copy loop to prevent findUniqueDestPath race conditions
- resolveManifestPaths walks manifest tree using pathMap; non-path strings pass through unchanged
- 33 tests passing

Author: alfonso-noriega