Clean-room, portable C++17 implementation of the PlanB IPv6 LPM algorithm.

Includes: - AVX-512 SIMD path + scalar fallback - Wait-free lookups with rebuild-and-swap dynamic FIB - Benchmarks on synthetic data and real RIPE RIS BGP (~254K prefixes)

Interesting result: on real BGP + uniform random lookups, a plain Patricia trie can sometimes match or beat the SIMD tree due to cache locality and early exits.

Would love feedback, especially comparisons with PopTrie / CP-Trie.