This change fixes delegation:
* If the Delegate object is not present, we should disallow empty permission list in DelegateSet preclaim.
* Empty permission list is only allowed to delete the existing Delegate object.
* In `doApply`, permission list being empty returns `tecINTERNAL`, which should not happen.
This change:
* Introduces a new helper function on `STTx`, `getFeePayer`.
* Removes the usage of `mSourceBalance` and replaces it with SLE balance lookups.
* Renames `mPriorBalance` to `preFeeBalance_`
This simplifies some of the code in the transactors and makes it a lot more readable.
This change reorganizes the `tx/transactors` directory for consistency and discoverability. There are no behavioral changes, this is a pure refactor. Underscores were chosen as the way to separate multi-words as this is the more popular option in C++ projects.
Specific changes:
- Rename all subdirectories to lowercase/snake_case (`AMM` → `amm`, `Check` → `check`, `NFT` → `nft`, `PermissionedDomain` → `permissioned_domain`, etc.)
- Merge `AMM/` and `Offer/` into `dex/`, including `PermissionedDEXHelpers`
- Rename `MPT/` → `token/`, absorbing `SetTrust` and `Clawback`
- Move top-level transactors into named groups: `account/`, `bridge/`, `credentials/`, `did/`, `escrow/`, `oracle/`, `payment/`, `payment_channel/`, `system/`
- Update all include paths across the codebase and `transactions.macro`
