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>

CHANGELOG.md

@@ -14,6 +14,7 @@
 
 - `StringKSUID.Equal` / `IsZero`, `BinaryKSUID.Equal` / `IsZero`, and nil sentinel vars for [ApplyInterfacePtr].
 - `Query`, `MustQuery`, and `UpdateOne` helpers (+ Context variants).
+- `WithTxValue` for transactional functions that return a value.
 - Context-aware variants for all query and mutation helpers.
 - `Delete`, `DeleteAffected`, `WithTx`, `ContainsCol`, and `CurrentDialect()`.
 - Package documentation (`doc.go`), expanded README, and subpackage docs for `dbxm` / `dbxp`.