fix: omit nftables interval end when range hits max address

[H-TTTTT]

Summary

• When building nftables interval sets, ranges whose upper bound is the address-family maximum (255.255.255.255 / ffff:...:ffff) made To().Next() invalid.
• The old fallback set the interval end equal to the start, so nftables rejected the duplicate key with EEXIST (file exists).
• Omit the end element in that case so the half-open interval extends through the max address (same convention as other nftables consumers).

Test plan

• route_exclude_address: ["ff00::/8"] with auto_redirect: true starts without EEXIST
• IPv4 240.0.0.0/4 / 224.0.0.0/3 exclude addresses also work
• Normal non-max ranges still encode start + exclusive end

Fixes SagerNet/sing-box#4316