fix(db): avoid passing computed orderBy aliases to loadSubset

[andrewcoelho]

🎯 Changes

TLDR: Fixes incorrect loadSubset optimization hints when a subquery orders by a computed selected field e.g. sortKey: coalesce(...).

I ran into this when trying to use an orderBy field that is a projected/computed alias. That was causing subset loading to use an invalid ordering hint, so the top-N window is built from the wrong subset and I observed query results that looked like limit was being applied before orderBy.

This change makes ref resolution stop at computed select expressions (only pass-through ref projections are followed), and only forwards orderBy hints when each orderBy ref resolves to a real source field.

• Updated followRef in packages/db/src/query/ir.ts to return undefined for computed selected fields.
• Applied the same followRef fix in packages/db/src/query/compiler/index.ts to keep compiler behavior consistent.
• Tightened computeSubscriptionOrderByHints in packages/db/src/query/live/utils.ts to validate each orderBy ref via followRef before passing orderBy/limit hints to loadSubset.
• Added a regression test in packages/db/tests/query/load-subset-subquery.test.ts to ensure computed subquery orderBy aliases do not forward orderBy/limit hints.

ETA sample query that shows the issue:

const pagedQuery = useLiveQuery(
  (q) => {
    // 1) Aggregate child records per parent (derived subquery)
    const latestChildByParent = q
      .from({ child: childCollection })
      .groupBy(({ child }) => child.parentId)
      .select(({ child }) => ({
        parentId: child.parentId,
        latestAt: max(child.createdAt),
      }))

    // 2) Join aggregate back to parent and compute a projected sort alias
    const parentsWithComputedSort = q
      .from({ parent: parentCollection })
      .join(
        { latest: latestChildByParent },
        ({ parent, latest }) => eq(parent.id, latest.parentId),
      )
      .select(({ parent, latest }) => ({
        id: parent.id,
        // computed/projection field (not a raw source column)
        computedSortKey: coalesce(latest.latestAt, parent.createdAt),
      }))

    // 3) Filter + order by computed alias + paginate
    return q
      .from({ row: parentsWithComputedSort })
      .where(({ row }) => gt(row.computedSortKey, anchorDate))
      .orderBy(({ row }) => row.computedSortKey, 'asc')
      .select(({ row }) => ({
        id: row.id,
        computedSortKey: row.computedSortKey,
      }))
      .limit(3)
  },
)

// Expected (ascending by computedSortKey):
// A
// B
// C

// Actual:
// A
// D
// E

// If I increase pageSize to include all rows:
// A
// B
// C
// D
// E
// ...

✅ Checklist

• I have tested this code locally with pnpm test.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).