Add comprehensive Phase 3 implementation guide

Created docs/PHASE3_GUIDE.md - Complete roadmap for Phase 3 development

Covers all Phase 3 goals:
1. wgpu-hal bridge for Bevy zero-copy integration
   - Two implementation approaches (direct hal vs render resource)
   - Format and usage mapping utilities
   - Key files and modifications needed

2. Bevy multi-window example
   - Concept: Two windows sharing one texture
   - Implementation outline with Bevy 0.14
   - Synchronization and lifecycle management

3. Bevy multi-process game
   - Architecture: Physics process + Display process
   - IPC with timeline semaphores
   - Real-world game engine use case

4. WebGPU/wgpu backend implementation
   - File structure and WgpuTextureShareManager
   - Native vs Web platform considerations
   - hal layer integration approach

5. Cross-API sharing with WebGPU
   - Vulkan ↔ wgpu sharing
   - Metal ↔ wgpu on macOS
   - Platform-specific strategies

6. Web platform support
   - Constraints and limitations
   - OffscreenCanvas, SharedArrayBuffer options
   - Pragmatic web implementation

Implementation Timeline:
- Phase 3.1: Bevy zero-copy (2-3 weeks)
- Phase 3.2: Bevy examples (1-2 weeks)
- Phase 3.3: WebGPU backend (3-4 weeks)
- Phase 3.4: Polish & docs (1 week)
Total: 8-12 weeks (part-time)

Includes:
- Technical challenges and solutions
- Resource links and community contacts
- Success criteria and next steps
- Code examples and architecture diagrams

Phase 3 Status: Ready to Begin
All Phase 2 prerequisites complete!

Author: vdmokstati