【性能调优】大包拆分：超长消息的切片与重组

---大消息的切片发送：每片 1400 字节
local SLICE = 1400

function Chunk.send(connId, msgId, payload)
    local total = math.ceil(#payload / SLICE)
    local seq = Chunk.nextSeq(connId)
    for i = 1, total do
        local from = (i - 1) * SLICE + 1
        local part = payload:sub(from, from + SLICE - 1)
        sendmsg(connId, msgId, table.concat({
            seq, ",", i, ",", total, ",", part,
        }))
    end
    return seq
end

---接收的重组：按序号的碎片拼装
Chunk.buffers = {}

function Chunk.receive(connId, header, part)
    local seq, i, total = header:match("(%d+),(%d+),(%d+)")
    local key = connId .. ":" .. seq
    local buf = Chunk.buffers[key]
    if not buf then
        buf = { parts = {}, got = 0, total = tonumber(total) }
        Chunk.buffers[key] = buf
    end
    if not buf.parts[i] then
        buf.parts[i] = part
        buf.got = buf.got + 1
    end
    -- 全部到齐：拼装并交付
    if buf.got == buf.total then
        local out = {}
        for j = 1, buf.total do
            out[j] = buf.parts[j]
        end
        Chunk.buffers[key] = nil
        Msg.deliver(connId, table.concat(out))
    end
end