[fslongjin]

Summary

• Vendor the smoltcp revision already used by the kernel and depend on it by path, so kernel builds no longer fetch that git revision.
• Keep the crate outside the kernel workspace so its tests and dev-dependencies stay out of the kernel lockfile.
• Add a workflow that runs formatting, MSRV clippy, feature tests, checks, and a short fuzz run only when this crate or its workflow changes.

Test plan

• Confirm the kernel still builds for x86_64 with the path dependency.
• Confirm the smoltcp workflow runs on changes under kernel/crates/smoltcp and does not run for unrelated changes.
• Confirm formatting, clippy, the feature test matrix, and the fuzz job pass on the vendored tree.

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