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

docs/product/multi-bank.md

@@ -90,9 +90,12 @@ reasons:
 
 So: `bank_book` is an ordered registry of `{ bank id, display name, ordinal,
 BankIndex }`, pool seeded as bank-zero. Bank id is the stable key (minted GUID-style
-on create); name and ordinal are mutable. `BankIndex` is untouched. This is the
-defer-the-feature, design-the-seam principle: the seam is a container above the
-tested core, not a modification of it.
+on create); name and ordinal are mutable. Display names are **unique** — two banks
+cannot share a name (compared trimmed + case-insensitively, so "Drums" and "drums"
+are the same name), enforced in the model on create and rename; the pool's "Pool" is
+reserved by the same rule. `BankIndex` is untouched. This is the defer-the-feature,
+design-the-seam principle: the seam is a container above the tested core, not a
+modification of it.
 
 ---
 
@@ -349,7 +352,9 @@ Mirrors the capture and Design View pillars exactly.
 - Ordered bank registry: `{ bank id, display name, ordinal, BankIndex }`; pool
   seeded with fixed id + fixed name.
 - Create / rename / reorder / delete named banks; pool-privilege rules enforced
-  here (reject delete-pool, reject rename-pool, never zero banks).
+  here (reject delete-pool, reject rename-pool, never zero banks). Display names are
+  unique — create/rename reject a name already used by another bank (trimmed +
+  case-insensitive; the pool's "Pool" is protected).
 - Active-bank id (get/set, defaults to pool); resolve the active bank's `BankIndex`.
 - Move / copy a sample between banks — index-only, destination collapse-by-hash
   observed, move removes the source entry.