import: a .rsbank lands as a new bank, whole or not at all

Four collisions answered explicitly: ids reminted, names never overwritten,
content deduped before the write, bank name auto-suffixed. Degraded ledger
refuses before the picker.

src/core/model/bank_book.cpp

@@ -117,6 +117,17 @@ bool BankBook::createBank(const std::string& id, const std::string& displayName)
     return true;
 }
 
+// Runs behind displayNameTaken so the probe and the create/rename check can never
+// disagree about what "already used" means. exceptId is deliberately "" — no bank can
+// carry an empty id, so nothing is excluded from the scan.
+std::string BankBook::uniqueDisplayName(const std::string& seed) const {
+    if (!displayNameTaken(seed, /*exceptId=*/std::string{})) return seed;
+    for (int n = 2;; ++n) {
+        std::string candidate = seed + " " + std::to_string(n);
+        if (!displayNameTaken(candidate, /*exceptId=*/std::string{})) return candidate;
+    }
+}
+
 bool BankBook::renameBank(const std::string& id, const std::string& displayName) {
     if (id == kPoolBankId) return false;  // pool is un-renamable
     Bank* b = bank(id);