feat: add filesystem snapshots and readonly zip mounts

[ivarvong]

Adds two filesystem capabilities on top of the VFS-backed JustBash filesystem.

Summary

• Add JustBash.FS.to_tree/2 to snapshot regular files under a root into a %{path => content} tree.
• Add JustBash.FS.Zip, an eager read-only VFS.Mountable backend for ZIP archives.
• Support from_binary/2 and from_url/3 ZIP construction, with URL fetches going through JustBash.Network and the configured HTTP client.
• Harden ZIP archive handling with structured VFS errors, unsafe path rejection, file/directory collision checks, empty directory support, UTF-8 paths, and construction limits.
• Add ZIP-specific POSIX behavior so mutation-like operations such as chmod fail with :erofs.
• Prepare output redirections before command execution so failed writes to read-only mounts prevent command side effects.
• Resolve relative HTTP redirects through the shared network policy.

Verification

• mix test test/just_bash/fs_zip_test.exs
• mix test
• mix compile --warnings-as-errors
• mix credo
• Live R2 smoke test against https://fieldvault.2fsk.com/zip_mount_demo.zip

Notes

JustBash.FS.Zip.from_url/3 enforces archive limits before decompression, after the configured HTTP client returns the response body. A future streaming HTTP client path could enforce :max_archive_bytes before materializing remote response bytes.