Fix LpVec3 entity velocity decoding (big-endian word + notch units)

[atiweb]

Problem

src/datatypes/lpVec3.js (added in #1453 for the 1.21.9+ entity-velocity encoding) decodes every entity velocity incorrectly. Two separate issues:

1. Wrong endianness. The 32-bit word is read with readUInt32LE and written with a little-endian writeUInt32LE/writeUInt16LE layout, but Minecraft's net.minecraft.network.LpVec3 reads/writes it with FriendlyByteBuf.readUnsignedInt / writeInt, which are big-endian. The decoded packet is still the correct length, so no parse error is raised — the velocity is just silently wrong. The existing test only covers the zero vector, which is a single 0 byte and therefore endian-agnostic, so this was never caught.

2. Wrong units. entity_velocity/spawn_entity use lpVec3 on >= 1.21.9 and vec3i16 on older versions, and consumers apply fromNotchVelocity (÷ 8000) to packet.velocity uniformly (e.g. mineflayer lib/plugins/entities.js, PrismarineJS/mineflayer#3839). vec3i16 yields raw 1/8000-block-per-tick units, but lpVec3 yielded raw blocks-per-tick — i.e. 8000× too small.

Together these break entity velocity / knockback on 1.21.9–1.21.11. See PrismarineJS/mineflayer#3887 ("Bot freezes in mid-air after taking knockback on 1.21.11").

Fix

• Read and write the 32-bit word big-endian, and rewrite the write path so the bytes match the on-wire layout packed = c << 16 | b << 8 | a (byte, byte, int32 big-endian).
• Scale the decoded vector to 1/8000-block-per-tick units to match vec3i16, so the existing fromNotchVelocity consumers work unchanged across all versions.

Validation

• New test/lpVec3Test.js decodes and round-trips velocity payloads captured from a real vanilla 1.21.11 server (e.g. f9ff7ffeebed → ≈ (0, -0.078, 0) blocks/tick) plus the zero-vector single-byte case.
• Verified live against a 1.21.11 server with mineflayer: a piglin brute's knockback now decodes to (0.087, 0.177, -0.258) blocks/tick and the bot takes knockback normally instead of freezing in mid-air.

Reference: net.minecraft.network.LpVec3.read = readUnsignedByte, readUnsignedByte, readUnsignedInt; the resulting Vec3 is the entity's blocks-per-tick movement.