perf: optimize incremental generator performance

[Skymly]

Part of #11

Summary

Incremental performance improvements for the ObservableEvents generator.

Changes

• Add ObservableEventTargetSets.Empty singleton to avoid repeated empty array allocations
• Cache Avalonia type metadata lookups (RoutedEvent and non-generic) to avoid repeated GetTypeByMetadataName calls
• Use static readonly field for common empty return path when bootstrap type is not found

Performance Impact

• Reduces allocations for projects not using Avalonia (cached detection)
• Faster empty path when bootstrap extensions are not referenced
• Minor CPU reduction for large candidate sets

Tests

• All 14 tests pass
• No functional changes