Canonicalize chunk units before extraction

docs/internal/modules.md

@@ -49,6 +49,10 @@ The `generic` chunker splits source units into ordered chunks. It validates the
 source document, clones source units, assigns chunk IDs such as `chunk-000001`,
 and records chunk metadata for start unit, end unit, and unit count.
 
+The pipeline runner canonicalizes chunk units from the source document by ID
+before extractors and mergers run. Chunker-owned context should stay in
+`SourceChunk.Metadata`.
+
 Options:
 
 - `max_units`: positive integer, default `50`;