feat(examples): add UnixFS builder example with HAMT sharding

[PsychoPunkSage]

Closes #663

This PR adds a new example demonstrating how to programmatically create UnixFS DAGs with features similar to ipfs add. The example shows how to:

• Create UnixFS structures from files and directories
• Handle large directories using HAMT sharding
• Preserve file metadata (mtime, mode)
• Customize chunking strategies
• Display progress with human-readable sizes

The example includes proper error handling, tests, and documentation for common use cases. This helps developers understand how to work with UnixFS programmatically using boxo's lower-level APIs.