feat: improve GraphQL type generation for lexicons
## Union Types
- Parse `refs` field from properties for union types
- Generate union types for property and array item unions
- Implement $type-based runtime type resolution
- Use contextual naming: `{ParentType}{Field}Union`
## Object Types
- Build object types from lexicon `#fragment` definitions
- Sort refs to build dependencies first (topological ordering)
- Add placeholder field for empty object types
- Expand local refs (#mention -> full NSID)
## Blob Handling
- Inject DID into blob objects for CDN URL generation
- Propagate DID through nested objects and arrays
## Array Fields
- Expand shorthand refs in array items
- Resolve array item refs to proper GraphQL types
- Handle union arrays with multiple item types
## Testing & Examples
- 199 tests for new functionality
- Add 02-following-feed example
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