Draft: xet CLI utility for file upload, download, and inspection

[hoytak]

This PR adds a new xet command-line binary to xet_pkg for directly uploading, downloading, and inspecting files against a CAS endpoint — useful for development, debugging, and scripting without going through git-xet or huggingface_hub.

The binary exposes four subcommands under xet file:

upload — upload one or more files (or stdin) and emit file metadata (hash, size, sha256).
download — download by xet hash to a file or stdout, with optional source and write byte ranges.
scan — dry-run dedup/compression analysis without uploading data.
dump-reconstruction — fetch and display reconstruction metadata as JSON.

Endpoint resolution currently follows the same conventions as session API, but using arguments passed in. --endpoint overrides HF_ENDPOINT, which defaults to https://huggingface.co; endpoint can also be a local directory in which case a LocalClient is used. Token resolution uses --token then HF_TOKEN.

All config values can be overridden with -c KEY=VALUE.

# Upload a single file and see its hash/size/sha256
xet file upload mydata.bin

# Upload multiple files at once
xet file upload file1.bin file2.bin file3.bin

# Upload from stdin
cat model.safetensors | xet file upload -

# Upload and write results to JSON
xet file upload --output results.json *.bin

# Download a file by hash
xet file download abc123...def -o restored.bin 

# Download a byte range to stdout
xet file download abc123...def --source-range 0..4096

# Download a range and write it into a specific offset of an existing file
xet file download abc123...def -o target.bin --source-range 1024..2048 --write-range 0..1024

# Dry-run scan to check dedup/compression ratio without uploading
xet file scan --recursive ./dataset/

# Inspect reconstruction metadata for a file hash
xet file dump-reconstruction abc123...def

# Use a local CAS directory instead of a remote endpoint
xet --endpoint /tmp/local-cas file upload data.bin