Improve handle management

[jaybosamiya-ms]

This PR merges some operations to use a single Handle enum, rather than do them individually. This simplifies the Backend interface and also allows for supporting chmod of / (for example), which is needed as more backends move to the new design.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-item-signature
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/trait_method_missing.ron

Failed in:
  method file_status of trait Backend, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/cc47dfa84e80a9c212344bff43023afd2f14fe76/litebox/src/fs/backend.rs:130
  method dir_status of trait Backend, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/cc47dfa84e80a9c212344bff43023afd2f14fe76/litebox/src/fs/backend.rs:133
  method chmod_at of trait Backend, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/cc47dfa84e80a9c212344bff43023afd2f14fe76/litebox/src/fs/backend.rs:154
  method chown_at of trait Backend, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-main/cc47dfa84e80a9c212344bff43023afd2f14fe76/litebox/src/fs/backend.rs:157

[wdcui]

LGTM. Thanks. GPT-5.6 sol reported the following potential issue:

Medium —  litebox/src/fs/resolver.rs:211-217, 653-676 

 chmod / chown  now require execute permission on the target directory itself. An owner cannot restore  /locked  from mode  0600  to  0700 , although only the parent should require search permission. Exclude the final target directory from walk-permission checks for metadata operations.

The concern was:

• Previously,  chmod("/a/locked")  walked only  /a .
• Now  path_handle()  walks  /a/locked .
• For a directory,  check_walk_permissions()  checks execute permission on  locked  too.
• POSIX requires search permission on parent directories, not the final directory being changed.

[jaybosamiya-ms]

Hmm, this is a valid concern, will fix up in a separate PR