axiom-sqlitedata-migration

# Migrating from SwiftData to SQLiteData

## When to Switch

```
┌─────────────────────────────────────────────────────────┐
│ Should I switch from SwiftData to SQLiteData?           │
├─────────────────────────────────────────────────────────┤
│                                                         │
│  Performance problems with 10k+ records?                │
│    YES → SQLiteData (10-50x faster for large datasets)  │
│                                                         │
│  Need CloudKit record SHARING (not just sync)?          │
│    YES → SQLiteData (SwiftData cannot share records)    │
│                                                         │
│  Complex queries across multiple tables?                │
│    YES → SQLiteData + raw GRDB when needed              │
│                                                         │
│  Need Sendable models for Swift 6 concurrency?          │
│    YES → SQLiteData (value types, not classes)          │
│                                                         │
│  Testing @Model classes is painful?                     │
│    YES → SQLiteData (pure structs, easy to mock)        │
│                                                         │
│  Happy with SwiftData for simple CRUD?                  │
│    YES → Stay with SwiftData (simpler for basic apps)   │
│                                                         │
└─────────────────────────────────────────────────────────┘
```

---

## Pattern Equivalents

| SwiftData | SQLiteData |
|-----------|------------|
| `@Model class Item` | `@Table nonisolated struct Item` |
| `@Attribute(.unique)` | `@Column(primaryKey: true)` or SQL UNIQUE |
| `@Relationship var tags: [Tag]` | `var tagIDs: [Tag.ID]` + join query |
| `@Query var items: [Item]` | `@FetchAll var items: [Item]` |
| `@Query(sort: \.title)` | `@FetchAll(Item.order(by: \.title))` |
| `@Query(filter: #Predicate { $0.isActive })` | `@FetchAll(Item.where(\.isActive))` |
| `@Environment(\.modelContext)` | `@Dependency