feat(query): add Query, MustQuery, and UpdateOne helpers

Adds scan-into-dest query helpers and update-one-row semantics with
Context variants, tests, and documentation updates.

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

exec.go

@@ -74,6 +74,24 @@ func UpdateAffectedContext(ctx context.Context, sqlo Executable, stmt Statement)
 	return rowsAffected, nil
 }
 
+// UpdateOne executes an update statement and returns notFoundErr if zero rows
+// were affected.
+func UpdateOne(sqlo Executable, stmt Statement, notFoundErr error) error {
+	return UpdateOneContext(context.Background(), sqlo, stmt, notFoundErr)
+}
+
+// UpdateOneContext is the context-aware variant of [UpdateOne].
+func UpdateOneContext(ctx context.Context, sqlo Executable, stmt Statement, notFoundErr error) error {
+	n, err := UpdateAffectedContext(ctx, sqlo, stmt)
+	if err != nil {
+		return err
+	}
+	if n == 0 {
+		return notFoundErr
+	}
+	return nil
+}
+
 // UpdateReturning executes an update statement that returns the updated row.
 // The statement MUST be a Jet UpdateStatement with a RETURNING clause. Returns
 // the updated row object T or an error if the update fails or no rows are returned.