Implement LibraryWriter

[leogdion]

Phase

Phase 3: Input/Output Handlers

Specification

Create library writer that writes LibraryUpdateResult to output directory with proper structure.

Acceptance Criteria

• LibraryWriter struct created with outputPath property
• writeUpdatedLibrary() creates output directory with intermediate directories
• Updated files written to correct paths
• New files written to correct paths
• Optional: unchanged files copied if needed
• Code follows one-type-per-file organization

Dependencies

• Depends on: Issue 12 (to be created)

Files to Create/Modify

• Sources/AiSTKit/LibraryWriter.swift

Related Documentation

• skit-analyze-plan.md