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>

doc.go

@@ -11,7 +11,7 @@
 // [Fetch], [FetchOne], [Query], [Insert], [Update], [Delete], and their Must*
 // and Context variants wrap Jet [Statement] execution with consistent error
 // semantics. [UpdateOne] requires at least one row affected.
-// [WithTx] runs a function inside a SQL transaction.
+// [WithTx] and [WithTxValue] run functions inside SQL transactions.
 //
 // # Jet column utilities
 //