feat(kebab-core): p9-fb-23 task 2 — CanonicalDocument gains last_chunker_version + last_embedding_version

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-chunk/src/md_heading_v1.rs

@@ -477,6 +477,8 @@ mod tests {
             parser_version: kebab_core::ParserVersion("test-parser-0".into()),
             schema_version: 1,
             doc_version: 1,
+            last_chunker_version: None,
+            last_embedding_version: None,
         }
     }