Refactor Ref System with Nested Optional Properties

.changeset/nested-optional-properties.md

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+---
+"@tanstack/db": patch
+---
+
+## Enhanced Ref System with Nested Optional Properties
+
+Comprehensive refactor of the ref system to properly support nested structures and optionality, aligning the type system with JavaScript's optional chaining behavior.
+
+### ✨ New Features
+
+- **Nested Optional Properties**: Full support for deeply nested optional objects (`employees.profile?.bio`, `orders.customer?.address?.street`)
+- **Enhanced Type Safety**: Optional types now correctly typed as `RefProxy<T> | undefined` with optionality outside the ref
+- **New Query Functions**: Added `isUndefined`, `isNotUndefined`, `isNull`, `isNotNull` for proper null/undefined checks
+- **Improved JOIN Handling**: Fixed optionality in JOIN operations and multiple GROUP BY support
+
+### ⚠️ Breaking Changes
+
+**IMPORTANT**: Code that previously ignored optionality now requires proper optional chaining syntax.
+
+```typescript
+// Before (worked but type-unsafe)
+employees.profile.bio // ❌ Now throws type error
+
+// After (correct and type-safe)
+employees.profile?.bio // ✅ Required syntax
+```
+
+### Migration
+
+Add `?.` when accessing potentially undefined nested properties