tcp_new_isn last_offset signed int overflow-wrap on churn (UB / monotonicity erosion)

Summary

struct tcp_isn.last_offset(:1674) is int accumulated += 0x10000000/reseed + ~0x100000/mint. Signed overflow UB after ~2K mints. tcp_isn_reseed_interval root-writable negative -> huge unsigned threshold disabling reseeds. ISN quality degradation.