feat(tx): add WithTxValue for transactional return values

WithTxValue runs a callback inside a transaction and returns its result,
with rollback on error. Includes tests and documentation updates.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -44,7 +44,7 @@ For Postgres, use `dbx.DialectPostgres` and blank-import `dbxp` instead of `dbxm
 |------|-----------|
 | Query | `Fetch`, `MustFetch`, `FetchOne`, `MustFetchOne`, `Query`, `MustQuery` (+ `*Context` variants) |
 | Mutations | `Insert`, `InsertReturning`, `Update`, `UpdateAffected`, `UpdateOne`, `UpdateReturning`, `Delete`, `DeleteAffected` (+ `*Context` variants) |
-| Transactions | `WithTx` |
+| Transactions | `WithTx`, `WithTxValue` |
 | Columns | `NormalCols`, `ContainsCol`, `ExprValues`, `ExprStringers` |
 | Partial update | `ApplyPtr`, `ApplyComplexPtr`, `ApplyInterfacePtr`, `ApplyVal` |
 | Pointers | `Ptr`, `Val`, `NowPtr`, `TrimPtr`, `TrimPtrToNil`, `IsZero` |