Reorganize transactors into feature subdirectories

- Rename subdirs to lowercase (AMM→amm, Check→check, NFT→nft, etc.)
- Merge AMM and Offer into dex/, add PermissionedDEXHelpers
- Rename mpt→token, add SetTrust and Clawback
- Group: account, bridge, oracle, payment, system, escrow, did,
  credentials, payment_channel, permissioned_domain, vault, lending

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

src/test/app/Batch_test.cpp

@@ -13,7 +13,7 @@
 #include <xrpl/protocol/jss.h>
 #include <xrpl/server/NetworkOPs.h>
 #include <xrpl/tx/apply.h>
-#include <xrpl/tx/transactors/Batch.h>
+#include <xrpl/tx/transactors/system/Batch.h>
 
 namespace xrpl {
 namespace test {