fix(db): preserve union types in select result extraction

[nathan-muir]

Fixes #1511

🎯 Changes

When a collection field is a discriminated union of object types, selecting that field via .select() collapses the union into a single type with only the intersection of keys.

type Document =
  | { type: 'pdf'; url: string; pages: number }
  | { type: 'image'; url: string; width: number; height: number }
  | { type: 'legacy'; path: string }

type Item = { id: number; document: Document }

// Before: result.document is { type: string } — union collapsed, variant keys lost
// After:  result.document is Document — full discriminated union preserved
q.from({ i: items }).select(({ i }) => ({ id: i.id, document: i.document }))

The root cause is in ResultTypeFromSelect: when a Ref<T> holds a union type, ExtractRef recurses into the mapped keys via keyof/Prettify, which collapse the union to only common keys.

The fix detects when a Ref's branded type (RefLeaf<T>) is a union and extracts it directly, bypassing ExtractRef. This preserves Ref<T>'s mapped type structure so that common keys of the union remain accessible in .where() and .orderBy() callbacks (e.g., eq(i.document.type, 'pdf')).

New helpers:

• IsUnion<T> — detects union types
• ExtractRefBrand<T> — extracts the raw type from a RefLeaf's brand

Tests added:

• Select preserves union types on object fields
• Where clause works on common keys of union fields
• Select preserves union when collection type is a union

✅ 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).