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

tests/test_bank_book.cpp

@@ -92,6 +92,8 @@ static void testCreateRenameReorder() {
     CHECK(book.renameBank("b", "Beta-renamed"));
     CHECK(book.bank("b")->displayName == "Beta-renamed");
     CHECK(!book.renameBank("missing", "x"));
+    // Renaming back to a non-colliding name keeps working.
+    CHECK(book.renameBank("b", "Beta"));
 
     // Reorder: move "c" to the front of the named region (ordinal 1).
     CHECK(book.reorderBank("c", 1));
@@ -113,6 +115,56 @@ static void testCreateRenameReorder() {
     CHECK(!book.reorderBank("missing", 1));
 }
 
+static void testDisplayNameUniqueness() {
+    BankBook book;
+    CHECK(book.createBank("a", "Drums"));
+
+    // A unique name is accepted.
+    CHECK(book.createBank("b", "Bass"));
+
+    // Exact duplicate rejected, no mutation (size unchanged, the collided id absent).
+    CHECK(!book.createBank("c", "Drums"));
+    CHECK(book.bank("c") == nullptr);
+    CHECK(book.size() == 3);  // pool + a + b only
+
+    // Trimmed + case-insensitive collisions: "drums", " Drums ", "DRUMS" all collide.
+    CHECK(!book.createBank("c", "drums"));
+    CHECK(!book.createBank("c", " Drums "));
+    CHECK(!book.createBank("c", "DRUMS"));
+    CHECK(book.bank("c") == nullptr);
+
+    // The pool's reserved name "Pool" (and its variants) cannot be taken by a new bank.
+    CHECK(!book.createBank("c", "Pool"));
+    CHECK(!book.createBank("c", "  pool  "));
+    CHECK(book.bank("c") == nullptr);
+
+    // -- renameBank uniqueness --------------------------------------------------
+    // Rename to a name used by ANOTHER bank is rejected (no mutation).
+    CHECK(!book.renameBank("b", "Drums"));
+    CHECK(book.bank("b")->displayName == "Bass");  // unchanged
+    CHECK(!book.renameBank("b", "drums"));         // case-insensitive collision too
+    CHECK(!book.renameBank("b", " Drums "));       // trimmed collision too
+
+    // Renaming a bank to its OWN current name is a no-op success (not a rejection).
+    CHECK(book.renameBank("a", "Drums"));
+    CHECK(book.bank("a")->displayName == "Drums");
+    // Re-casing/-spacing its own name is likewise allowed (it collides only with self).
+    CHECK(book.renameBank("a", "  drums "));
+    CHECK(book.bank("a")->displayName == "  drums ");
+
+    // Renaming to the pool's reserved name is rejected (pool is the "other" bank here).
+    CHECK(!book.renameBank("b", "Pool"));
+    CHECK(book.bank("b")->displayName == "Bass");
+
+    // A genuinely fresh unique name still renames fine.
+    CHECK(book.renameBank("b", "Low End"));
+    CHECK(book.bank("b")->displayName == "Low End");
+
+    // After the rejections, the previously-freed name is now reusable by a new bank.
+    CHECK(book.createBank("c", "Bass"));
+    CHECK(book.bank("c")->displayName == "Bass");
+}
+
 static void testMoveSourceLosesDestGains() {
     BankBook book;
     CHECK(book.createBank("drums", "Drums"));
@@ -473,6 +525,7 @@ int main() {
     testPoolSeededAndDefaults();
     testPoolPrivileges();
     testCreateRenameReorder();
+    testDisplayNameUniqueness();
     testMoveSourceLosesDestGains();
     testCopySourceRetainedDestGains();
     testMoveDestCollapse();