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_test.go

@@ -60,6 +60,29 @@ func TestUpdateAffected_ReturnsCount(t *testing.T) {
 	assert.Equal(t, int64(3), n)
 }
 
+func TestUpdateOne_NotFound(t *testing.T) {
+	errNotFound := errors.New("not found")
+	stmt := mockStatement{
+		execContextFn: func(_ context.Context) (sql.Result, error) {
+			return mockResult{rowsAffected: 0}, nil
+		},
+	}
+
+	err := UpdateOne(mockExecutable{}, stmt, errNotFound)
+	assert.ErrorIs(t, err, errNotFound)
+}
+
+func TestUpdateOne_Success(t *testing.T) {
+	stmt := mockStatement{
+		execContextFn: func(_ context.Context) (sql.Result, error) {
+			return mockResult{rowsAffected: 1}, nil
+		},
+	}
+
+	err := UpdateOne(mockExecutable{}, stmt, errors.New("not found"))
+	require.NoError(t, err)
+}
+
 func TestDelete_Succeeds(t *testing.T) {
 	called := false
 	stmt := mockStatement{