Make the fs Backend dyn-compatible

[jaybosamiya-ms]

#887 introduced the Backend abstraction to the file system, but as I was working on more functionality, I discovered that the better design is to guarantee that Backend is dyn-compatible, so that the composition of multiple backends can be done without a huge amount of type-level machinery + macros; it also keeps the interfaces more ergonomic for consumers of LiteBox core. To make things dyn-safe, we need to move out the Backend-specific handles; however, rather than the naive handling of this, there's a convenient abstraction such that backend implementations still end up with a largely convenient way to use their own typed handles.

[github-actions]

ℹ️ Note: This semver check was run against the jayb/fs-via-backend-part1 branch, not main or ulitebox.

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure trait_removed_associated_type: trait's associated type was removed ---

Description:
A public trait's associated type was removed or renamed.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/trait_removed_associated_type.ron

Failed in:
  associated type Backend::WalkingDirHandle, previously at /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_fs_via_backend_part1/5cac1175baf4dacc6c4032514216dc09aa5a1c25/litebox/src/fs/backend.rs:43
  associated type Backend::FileHandle, previously at /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_fs_via_backend_part1/5cac1175baf4dacc6c4032514216dc09aa5a1c25/litebox/src/fs/backend.rs:46
  associated type Backend::DirHandle, previously at /home/runner/work/litebox/litebox/target/semver-checks/git-jayb_fs_via_backend_part1/5cac1175baf4dacc6c4032514216dc09aa5a1c25/litebox/src/fs/backend.rs:49

[wdcui]

LGTM. Thanks!

GPT-5.5 reported the following two findings. Please double check them.

1. litebox/src/fs/resolver.rs: O_PATH is accepted, but normal operations on those fds hit unimplemented!(), so a caller can panic LiteBox instead of getting an errno-style error such as EBADF.

2. litebox/src/fs/devices.rs: The devices backend gives / and /dev the same (dev, ino): dev_dir_inode starts at inode 0, and root status also reports inode 0. This breaks inode identity for distinct directories.