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.h

@@ -100,6 +100,17 @@ public:
     // no-op success.
     bool renameBank(const std::string& id, const std::string& displayName);
 
+    // The first name in the sequence `seed`, "seed 2", "seed 3", … whose fold is free
+    // in this book — what a caller that must not be rejected (the package import) asks
+    // for before createBank. First-FREE-ascending, not highest-plus-one, so it fills a
+    // gap ("Drums" + "Drums 3" present yields "Drums 2") and is a pure function of the
+    // current name set. The seed is returned verbatim when free and is NEVER re-parsed:
+    // a bare trailing integer cannot be told from a user's own name, so "Kit 808" would
+    // become "Kit 2" under a stripping rule. Terminates by pigeonhole (one of the first
+    // N+1 candidates is free for N banks), so it needs no cap. A blank seed comes back
+    // blank — what a missing name should become is the caller's policy, not the model's.
+    std::string uniqueDisplayName(const std::string& seed) const;
+
     // Deletes a named bank and its member entries (files untouched — a shell/prune
     // concern). Rejects (false, no mutation) an unknown id or the pool. Remaining
     // ordinals compact after; if the deleted bank was active, falls back to the pool.