feat: sub-query support

[adrianbrowning]

Summary

Add ability to use sub-queries within the query builder — e.g. in WHERE, FROM, SELECT, and JOIN clauses.

Use Cases

• WHERE id IN (SELECT id FROM ...) — correlated/uncorrelated sub-queries
• FROM (SELECT ...) AS subquery — derived tables
• SELECT (SELECT COUNT(*) FROM ...) AS total — scalar sub-queries
• JOIN (SELECT ...) AS sub ON ... — sub-query joins

Proposed API (rough)

// WHERE IN sub-query
db.user.select(({ where, subQuery }) =>
  where({ id: { in: subQuery('post', q => q.select(({ field }) => [field('authorId')])) } })
)

// Derived table / FROM sub-query
db.user.select(({ fromSubQuery }) =>
  fromSubQuery('post', q => q.select(...), 'p')
)

Notes

• Sub-queries should be typed — return type inferred from inner select
• Must support both correlated and uncorrelated forms
• Consider parameterized vs. inlined values for safety