Make chunk validation a framework contract

docs/internal/modules.md

@@ -20,6 +20,11 @@ A production module package should provide:
 Module specs should describe capabilities accurately. Resolution uses specs to
 reject incompatible pipelines before execution.
 
+Chunk modules receive the structured LLM client through `contracts.ChunkRequest`
+when they need model-backed chunking. The pipeline runner validates generic
+chunk result invariants before extraction; module-owned policies may be stricter
+but must stay within the module package.
+
 ## `seriatim` Input
 
 Package: `internal/modules/input/seriatim`