Move chunks into the canonical source model

docs/internal/pipeline.md

@@ -77,7 +77,8 @@ normalize requests retain access to the original source material.
 Source validation requires every unit to carry a canonical self-reference to
 its containing document and its own unit ID. Explicit clone, checkpoint, and
 debug boundaries retain that reference, and the canonical source digest covers
-it deterministically.
+it deterministically. Chunks use the same source model and carry one canonical
+reference spanning the first selected unit through the last.
 
 `pipeline.RunOutput` carries the run manifest, accepted normalized results,
 rejected results, warnings, checkpoint events, and logical files returned by the
@@ -115,13 +116,15 @@ whose results are accepted and used.
 ## Chunk Canonicalization
 
 Before lane execution, generic validation requires unique chunk IDs, matching
-source identity, indexes matching returned order, valid ordered boundaries,
+source identity, indexes matching returned order, a valid canonical reference,
 non-empty content and media type, and at least one valid source unit per chunk.
-Units may not repeat inside a chunk and must preserve source-document order.
+Units may not repeat inside a chunk and must form a contiguous range in
+source-document order. The chunk reference must exactly match the source and
+the first and last unit references.
 
 The runner then rebuilds each chunk's unit slice from the source document by
-unit ID. It preserves the module-owned boundaries, content, media type, and
-cloned metadata. The framework permits gaps and overlap between separate
+unit ID. It preserves the canonical reference, content, media type, and cloned
+metadata. The framework permits gaps and overlap between separate
 chunks; stricter coverage policy belongs to the chunk implementation.
 
 ## Validation And Retries