fix(parse): keep the alias when an expression continues past a group

[tiagolauer]

Fixes #290.

The bug

Query<DB, 'select (id + 1) * 2 as x from users'>
// { '* 2 as x': unknown }[]

Expected { x: unknown }[] — the value being unknown is fine, the key is the bug.

The fix

The paren-entry branch assumed whatever follows the closing paren is an alias and returned [Trim<After>, Expr] directly. The bare-entry branch guards the same situation with IsOperatorExpression; this branch had no such check.

When the tail is operator-shaped, the entry is now split at its top-level AS — the same signal the rest of the parser uses — so (id + 1) * 2 as x gives alias x and expression (id + 1) * 2.

Without an AS there is nothing to tell an alias from the rest of the expression, so the whole entry becomes the key ('(id + 1) * 2 x'). That is the answer the bare-entry branch gives for the same shape, and better than the old '* 2 x', which named a fragment. That case is pinned by a test.

Verification

tests/paren-expression-alias.test-d.ts, eight cases. Four red on master:

tests/paren-expression-alias.test-d.ts(17,3): error TS2344: Type 'false' does not satisfy the constraint 'true'.
tests/paren-expression-alias.test-d.ts(25,3): ...
tests/paren-expression-alias.test-d.ts(29,3): ...
tests/paren-expression-alias.test-d.ts(33,3): ...

• * 2 as x, / 2 as average, the no-AS form, and one among ordinary columns

Controls: (id + 1) as next, (id + 1) next, a scalar subquery with an alias, and count(*) as total are all unchanged.

tsc --noEmit clean, budget 192,636 (+150).