nvx-rs: give user IDs and file modes distinct types in the Linux platform primitives #643

Follow-up to #628, from its review. It waits for #628 to merge.

Summary

store_dir_problem in nvx-rs/src/host/platform/linux.rs takes (uid: u32, mode: u32, euid: u32). Two of the three are user IDs of the same type, so a caller that swaps the owner and the current user, or passes a mode as a user ID, still compiles, and the privacy check then compares the wrong values. Wrap user IDs and modes in distinct types so the compiler rejects such mistakes.

Proposal

• Use distinct types for a user ID and a file mode in the Linux platform primitives that check ownership and permissions. nix, already a dependency of the host feature, provides nix::unistd::Uid and nix::sys::stat::Mode; a small newtype of the crate's own is fine too.
• Convert at the boundary where std::os::unix::fs::MetadataExt returns raw integers, and keep raw integers out of the signatures of the checks.
• Keep the messages, which print user IDs in decimal and modes in octal, unchanged.
• Apply the same types to other places in platform/linux.rs that pass user IDs or modes as bare integers.

Acceptance criteria

• Passing a mode where a user ID is expected, or the reverse, fails to compile.
• The unit tests of store_dir_problem and the store's privacy tests pass unchanged in what they assert.
• No behavior changes.

Validation

On Linux, cargo clippy with --all-targets --all-features -- -D warnings and cargo test --all-features for nvx-rs/Cargo.toml. CI runs the rest of check-nvx-rs. The change needs no hypervisor.