Fix: cargo clippy -- -D warnings on macOS

[SSakutaro]

Summary

Fixes cargo clippy -- -D warnings on macOS.

The macOS build reported:

• unused imports
• unused variable
• needless return

Linux-only imports and variables are now guarded with #[cfg(target_os = "linux")], and needless return statements were removed from platform-specific socket creation branches.

Testing

Ran the following checks on Linux and macOS:

• cargo build
• cargo test
• cargo clippy -- -D warnings
• cargo fmt -- --check

[Copilot]

Pull request overview

This PR cleans up platform-specific ICMP probing code so the project passes cargo clippy -- -D warnings on macOS without changing the intended runtime behavior. The changes fit the existing cross-platform socket/probing code by narrowing Linux-only imports/variables and simplifying a few platform-gated return paths.

Changes:

• Guard Linux-only probe-engine imports and the is_dgram local with #[cfg(target_os = "linux")] so macOS builds no longer report them as unused.
• Remove a few needless return statements in OS-specific socket construction branches flagged by Clippy.
• Keep the platform-specific send/receive socket behavior unchanged while making the code compile cleanly across supported Unix targets.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/trace/engine.rs	Narrows Linux-only imports and locals in ICMP probe execution to match actual platform usage.
src/probe/socket.rs	Simplifies several cfg-gated socket factory branches by returning expressions directly instead of using explicit return.

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[lance0]

Thanks @SSakutaro — clean fix. The Linux cfg-gating and dropping the redundant returns on cfg-isolated branches is exactly right.

You also caught a CI gap on our end: clippy only runs on Linux, so macOS-only warnings are invisible in our own PRs. Adding clippy to build-macos and build-freebsd in a follow-up — bug catches CI bug, nice work. Merging.

[SSakutaro]

@lance0
Thanks for the quick review and merge!
Happy to help catch the CI gap. I'll proceed with the Homebrew core PR next.

[lance0]

No problem ! Appreciate you.