journal: archive importer — propagate merge log/staging paths, summary errors, and segment_errored decisions
- Adds merge_log_path and merge_staging_path to ImportResult; populated from _merge_artifact_paths() and mirrored into processing_results and the importer.file_imported Callosum payload.
- Persists merge summary_errors so the UI can surface them later without re-reading source files.
- Adds staging_path to entity_staged decision rows; emits segment_errored decisions on per-segment failures.
Co-Authored-By: OpenAI Codex <codex@openai.com>