docs(cheatsheet): 添加Lua语法速查表文档

新增Lua语法速查表，包含变量、控制结构、表操作、函数定义及常用标准库等内容，方便开发者快速查阅Lua语法要点

Author: mudssky

docs/cheatsheet/lua/lua_cheatsheet.md

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+# Lua 语法速查表 (Cheatsheet)
+
+## 1. 变量与基本数据类型
+
+- 推荐：使用局部变量，避免污染全局作用域
+
+```lua
+local a = 10
+local b = true
+local s = "hello"
+local t = { 1, 2, 3 }
+local f = function(x) return x * 2 end
+local n = nil
+```
+
+- 基本类型：`nil`、`boolean`、`number`、`string`、`table`、`function`
+
+## 2. 控制结构
+
+- if-then-elseif-else
+
+```lua
+local x = 5
+if x > 10 then
+    print("big")
+elseif x > 5 then
+    print("medium")
+else
+    print("small")
+end
+```
+
+- while 循环
+
+```lua
+local i = 0
+while i < 3 do
+    print(i)
+    i = i + 1
+end
+```
+
+- repeat-until 循环
+
+```lua
+local i = 0
+repeat
+    i = i + 1
+until i == 3
+```
+
+- for 数值型循环
+
+```lua
+for i = 1, 5 do
+    print(i)
+end
+
+for i = 10, 1, -2 do
+    print(i)
+end
+```
+
+- for 泛型循环（遍历表）
+
+```lua
+local arr = { "a", "b", "c" }
+for i, v in ipairs(arr) do
+    print(i, v)
+end
+
+local dict = { name = "lua", version = 5.4 }
+for k, v in pairs(dict) do
+    print(k, v)
+end
+```
+
+## 3. 表（table）操作
+
+- 创建与初始化（数组与字典）
+
+```lua
+local arr = { 10, 20, 30 }
+local dict = { a = 1, b = 2 }
+```
+
+- 插入与删除（数组）
+
+```lua
+local arr = { 10 }
+table.insert(arr, 20)
+table.insert(arr, 1, 5)
+local last = table.remove(arr)
+local first = table.remove(arr, 1)
+```
+
+- 遍历
+
+```lua
+local arr = { 10, 20, 30 }
+for i, v in ipairs(arr) do
+    print(i, v)
+end
+
+local dict = { a = 1, b = 2 }
+for k, v in pairs(dict) do
+    print(k, v)
+end
+```
+
+- 删除字典键
+
+```lua
+local dict = { a = 1, b = 2 }
+dict.a = nil
+```
+
+- 数组与字典用法区分
+  - 数组：顺序索引从 `1` 开始，使用 `ipairs`、`table.insert`、`table.remove`
+  - 字典：键值对结构，使用 `pairs`，删除通过设为 `nil`
+
+## 4. 函数定义与调用
+
+- 基本定义与调用
+
+```lua
+local function add(x, y)
+    return x + y
+end
+
+local r = add(2, 3)
+```
+
+- 多返回值
+
+```lua
+local function divmod(a, b)
+    return math.floor(a / b), a % b
+end
+
+local q, m = divmod(10, 3)
+```
+
+- 可变参数（...）
+
+```lua
+local function sum(...)
+    local total = 0
+    for i = 1, select("#", ...) do
+        total = total + select(i, ...)
+    end
+    return total
+end
+
+local s = sum(1, 2, 3)
+```
+
+## 5. 常用标准库函数
+
+- 字符串处理（string）
+
+```lua
+local s = "hello lua"
+local len = #s
+local sub = string.sub(s, 1, 5)
+local found = string.find(s, "lua")
+local replaced = string.gsub(s, "lua", "world")
+local formatted = string.format("%s-%d", "id", 100)
+```
+
+- 表操作（table）
+
+```lua
+local arr = { "a", "b", "c" }
+local joined = table.concat(arr, ",")
+table.sort(arr)
+```
+
+- 基础 I/O（io）
+
+```lua
+for line in io.lines("input.txt") do
+    print(line)
+end
+
+local f = assert(io.open("out.txt", "w"))
+f:write("ok\n")
+f:close()
+```
+