`except_` drops explicit right-associated grouping

## Description

`SqlSetOperation` loses explicit right-associated grouping for `except_`.
Because SQL `EXCEPT` is not associative, this changes query results.

The Haskell functions currently have no fixity declarations, so an
unparenthesized chain defaults to left-associative parsing. More importantly,
even when the caller explicitly constructs a right-associated expression, the
renderer flattens the set-operation tree and omits the required SQL
parentheses.

## Behavioral reproduction

```haskell
itDb "preserves right-associated EXCEPT nesting" $ do
let singleton :: Int -> SqlQuery (SqlExpr (Value Int))
singleton n = pure $ val n
(result :: [Value Int]) <- select $ Experimental.from $
singleton 1 `except_` (singleton 1 `except_` singleton 1)
asserting $ result `shouldBe` [Value 1]
```

Expected result:

```haskell
[Value 1]
```

Actual result:

```haskell
[]
```

Semantically, the inner `1 EXCEPT 1` is empty, so the outer operation should
return `1`. The generated SQL is effectively evaluated as the left-associated
expression instead:

```sql
(SELECT 1 EXCEPT SELECT 1) EXCEPT SELECT 1
```

rather than:

```sql
SELECT 1 EXCEPT (SELECT 1 EXCEPT SELECT 1)
```

## Likely cause

`mkSetOperation` recursively renders both operands with the enclosing
`NeedParens` value and concatenates their clauses. A nested set operation does
not retain its position in the Haskell expression tree, so explicit grouping
is lost.

The fix should preserve nested set-operation grouping, particularly on the
right side of `except_` and in mixed-operator trees. It may also be worth
declaring/documenting the Haskell fixity of `except_` as non-associative so
chained uses require explicit grouping.

Found while reviewing #436; this issue is separate from that PR's identifier
allocation change and is reproducible on current `master`.

Start at mkSetOperation and the except_ entry point, then run the provided itDb reproduction on current master. Trace how nested operands are rendered and add or update coverage for right-associated EXCEPT and mixed-operator trees; done means explicit grouping is retained and the reproduction returns [Value 1].

Written by the indexing model from the issue text.