feat(banks): bindable multi-bank action family — create/rename/delete/evacuate/activate/move/copy + full-height toggles (B3)

src/bank_book.h

@@ -221,4 +221,17 @@ private:
     void adoptBanks(std::vector<Bank>&& banks, const std::string& activeBank);
 };
 
+// The next bank id to activate when cycling the active bank forward, in ordinal
+// order (the ids arrive pool-first, named 1..N, matching banks()). Wraps: the id
+// after the last returns the first (pool → named → … → pool). This is the pure
+// decision behind the "cycle active bank" action — the shell reads the book's
+// ordered bank ids + current active id, asks for the next, and activates it.
+//   * empty list                 -> "" (nothing to cycle to)
+//   * single id (pool-only)      -> that id (a one-bank book stays put)
+//   * currentBankId not present  -> the first id (a sane home to jump to)
+// Exposed as a free function (not a BankBook member) so it is unit-testable against
+// a bare id vector without a full book. Mirror of view_mode_model's nextModeId.
+std::string nextBankId(const std::vector<std::string>& orderedBankIds,
+                       const std::string& currentBankId);
+
 } // namespace reasampler