fix: book_chapter step5 rag_write source_step:4 — character-state extraction

Add source_step: 4 so rag_write explicitly targets PASS 3 output instead of
relying on last_text() which could fall back to the chapter document text.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

templates/book_chapter.yml

@@ -323,6 +323,7 @@ steps:
 
   - type: rag_write
     asset_id: "character-state"
+    source_step: 4   # PASS 3 character-state extraction; explicit to avoid last_text() falling back to chapter text
     max_chars: 4000
 
   - type: document