Extend image support with caching, in-memory data, z-index, and protocol selection

[meszmate]

• Add in-memory image rendering (raw RGB/RGBA/PNG bytes) for all protocols
• Add Kitty image caching (transmit once, display many) with cache/place/delete API
• Add z-index layering and unicode placeholders for Kitty protocol
• Add protocol override to force kitty/iterm2/sixel instead of auto-detection
• Add file validation before sending image commands
• Update README with full image API docs and usage examples
• Update hello_world example with interactive image feature demos
• Fix ImageCapabilities import path in context.zig