fix(banks): enforce unique bank display names in-model + B3 review minors

src/bank_book.h

@@ -97,10 +97,14 @@ public:
 
     // Creates a named bank with the caller-supplied stable id and display name,
     // assigning the next ordinal. Rejects (returns false, no mutation) an empty id,
-    // a duplicate id, or the reserved pool id. Display name is not required unique.
+    // a duplicate id, the reserved pool id, or a display name that duplicates an
+    // existing bank's name (including the pool's "Pool"). Display-name uniqueness is
+    // trimmed + case-insensitive (ASCII): "Drums", "drums", and " Drums " collide.
     bool createBank(const std::string& id, const std::string& displayName);
 
-    // Renames a named bank. Rejects (false, no mutation) an unknown id or the pool.
+    // Renames a named bank. Rejects (false, no mutation) an unknown id, the pool, or a
+    // target name already used by a DIFFERENT bank (trimmed + case-insensitive, as
+    // createBank). Renaming a bank to its own current name is a no-op success.
     bool renameBank(const std::string& id, const std::string& displayName);
 
     // Deletes a NAMED bank, removing it (and its member index entries) from the
@@ -210,6 +214,11 @@ private:
     std::vector<Bank> banks_;         // ordinal order; banks_[0] is always the pool
     std::string       activeBankId_;  // always names a live bank; defaults to pool
 
+    // 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.
+    bool displayNameTaken(const std::string& name, const std::string& exceptId) const;
+
     // Re-sorts banks_ by ordinal (pool pinned first) and rewrites ordinals to a
     // contiguous 0..N-1 so the pool is 0 and named banks are 1..N. Called after any
     // structural change (create / delete / reorder).