#34733 base red: clippy -D warnings in cmux-wg-tun utun.rs and acpmux repo_sync_tests.rs

Two clippy -D warnings errors are red at the feat-cmux-next tip. They fail the --arg all step of every Rust PR. For example, R-INPUT's #34641: its green step 5a79450ad2d0 passed 57 of 57 of its own tests and failed only on these two.

• cmux-tui/crates/cmux-wg-tun/src/utun.rs:32: std::mem::size_of is in the prelude. unused_qualifications is denied under -D warnings. Added by f18c96cb135 (#34248).
• cmux-tui/crates/acpmux/src/hub/permissions/repo_sync_tests.rs:11: a field is set right after Config::default(), which breaks clippy's field_reassign_with_default. Added by bf98a753624 (#34491). The test fixture now builds Config with ..Default::default(), as folder_profiles_tests.rs already does.

No behavior change. One is a test fixture, the other a path spelling.

Evidence: the workspace clippy step scripts/ci/cmux-tui-rust-check.sh --os linux --arg clippy at this head, step a03e4409634b56ecaaac2c55: running.

Observed / Not run

• Observed: the two errors in step 5a79450ad2d04812cd34cd68 at a feat-based head (log lines "unnecessary qualification" and "field assignment outside of initializer").
• Not run: nothing else; there is no behavior to test.

Changelog

none

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Summary by cubic

Fixes two clippy -D warnings errors that were failing the CI --arg all step on every Rust PR at the feat-cmux-next tip. No behavior change.

• utun.rs stops qualifying size_of with std::mem::, which is redundant since it's in the prelude and fails the unused_qualifications lint.
• repo_sync_tests.rs builds Config with ..Default::default() instead of assigning a field right after Config::default(), which fails the field_reassign_with_default lint.