Boat Standing Fix

Port to Minecraft 1.21.11 (Yarn mappings). A fresh re-implementation, not a rebuild: different mapping set, Loom/Gradle toolchain, and several renamed classes/methods (ServerPlayNetworkHandler, onPlayerMove, onVehicleMove, AbstractBoatEntity, Box, WorldView). Root-caused and verified directly against the real 1.21.11 merged server jar (javap/decompiling) that both checks this mod loosens have direct structural analogs to the 26.x version, including the same boat self-match risk in the vehicle-movement validation call site, which is guarded against from the start here. See the README for the full Mojmap-to-Yarn mapping table.