refactor(newappeal): extract payload cleanup helpers (Slice 2a)

- extracted boolean normalization

- extracted null key removal

- extracted internal key stripping

- no behaviour change

context/refactor-tracker.md

@@ -4,8 +4,8 @@ Base branch for this tracker: `refactor`
 
 ## Status
 
-Current slice: Slice 1 — XML/Form Derivation Extraction  
-Status: NOT STARTED
+Current slice: Slice 2b — File Merge/Dedupe Helpers  
+Status: READY TO START
 
 ---
 
@@ -13,7 +13,7 @@ Status: NOT STARTED
 
 ### Slice 1 — XML/Form Derivation Extraction
 
-Status: NOT STARTED
+Status: COMPLETE
 
 - Extract XML parsing helpers into lib
 - Keep selectors identical
@@ -23,7 +23,7 @@ Status: NOT STARTED
 
 ### Slice 2a — Payload Cleanup Helpers
 
-Status: NOT STARTED
+Status: COMPLETE
 
 - Extract boolean normalization
 - Extract null/internal key stripping