怪物分布：密度分层与点位轮换

-- 错误：固定一点集中刷，门口怪海深处空场
function spawnWave(map, n)
    for _ = 1, n do
        spawnmonster(map, 100, 120)
    end
end

local SPOTS = {
    {x = 100, y = 120, w = 5},
    {x = 220, y = 90,  w = 3},
    {x = 350, y = 260, w = 1},
}
local function pickSpot()
    local total = 0
    for _, s in ipairs(SPOTS) do
        total = total + s.w
    end
    local r = math.random(1, total)
    for _, s in ipairs(SPOTS) do
        r = r - s.w
        if r <= 0 then
            return s
        end
    end
    return SPOTS[1]
end