[UTILS] Implement `b2info` CLI tool for instant container inspection

[FrancescAlted]

Inspecting a .b2nd, .b2frame, .b2z, or .b2d file currently requires either launching the interactive b2view TUI or writing a custom Python script. When working in non-interactive terminal sessions, remote SSH shells without a full TTY, or automated CI pipelines, users need a fast, non-interactive CLI inspection tool (similar to h5dump -H, ncdump -h, or parquet-tools meta).

src/blosc2/info.py already implements human-readable formatting, but there is no CLI script entry point.

Suggested Work

1. Implement src/blosc2/cli/b2info.py and register it in pyproject.toml under [project.scripts]:

   b2info = "blosc2.cli.b2info:main"

2. Support inspecting all standard Blosc2 file formats: .b2nd, .b2z, .b2d, and .b2frame.
3. Provide command-line options:
   • Default output: Clean summary table (container type, shape, dtype, chunks, blocks, itemsize, uncompressed size, compressed size, compression ratio, codecs, filters).
   • --json: Output full metadata and layout as JSON for shell scripts and CI.
   • --meta: Display user metadata (meta and vlmeta dictionaries).
   • --tree: For TreeStore (.b2z/.b2d), print an ASCII group hierarchy.
4. Add unit tests under tests/test_cli_b2info.py.
5. Add a brief guide at doc/guides/b2info.md and link it in doc/guides/index.rst.