Unify TUN initialization and raw-IP writes, preserve socketpair directions, and bound reader delivery with queue backpressure. Make deferred packet ownership safe across close and uasync teardown, and apply per-interface MTU limits to lwIP MSS.
Validation: full check.sh passed 172 tests, 9 proxy integration checks, burst and load. TUN ASAN/UBSAN regressions passed 30 cases; TCP exchange passed with asymmetric and minimum MTUs. Sanitized NAT integration exposed an existing unaligned uint16_t read in eim_nat.c; that separate defect remains unchanged. Native Windows and FreeBSD validation remains outstanding.