Q-W5: persist → session/ext_state_io/prune_fs (deletion authority concentrated); one GetProjExtState grow-loop in bridge_marshal (T2-04, ×3 rewired); bank_book JSON codec → bank_book_json via private static nameKey; persist.h stays umbrella. 61/61 green.

src/core/model/bank_book.h

@@ -342,6 +342,16 @@ private:
     std::vector<Bank> banks_;         // ordinal order; banks_[0] is always the pool
     std::string       activeBankId_;  // always names a live bank; defaults to pool
 
+    // Folds a display name to its uniqueness key: strip leading/trailing ASCII
+    // whitespace, lower-case ASCII letters. So "Drums", "drums", and " Drums " share
+    // one key and cannot coexist. ASCII-only by design — the pure core carries no
+    // locale facility and must not grow one. A private STATIC member (Q-W5, settled)
+    // because BOTH halves of the split implementation need the ONE folding rule: the
+    // rules TU (bank_book.cpp, displayNameTaken) and the JSON TU (bank_book_json.cpp,
+    // deserialize's duplicate-display-name coalesce) — a drifted second copy would let
+    // a parsed book violate the create/rename uniqueness invariant.
+    static std::string nameKey(const std::string& s);
+
     // True if a bank OTHER than `exceptId` already carries `name`'s uniqueness key
     // (trimmed + case-insensitive, ASCII). Backs the create/rename uniqueness check;
     // pass exceptId=id to let a bank keep (or re-case/-space) its own name.