fix(S5-03): 改型卡的累加例子改成真的输出负数，并绑上护栏 - #67
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任务 2 的改型卡拿一个累加例子讲 int 回绕，题面写的是「手算总和是正数， 程序却输出负数」。两个历史版本的数字本机 g++ 实测都回绕成正数： - 「累加 1 到 100000 再乘系数 100000」-> +2087268864 - 「n=100000，每项 10^5」-> +1410065408 孩子照着跑看不到任何负数，会以为「没有溢出」。同一个说法还出现在三步对照卡 与脚手架注释里。 例子改成 n=100000、每项 80000：手算总和 8000000000（80 亿，正数），int 实测 输出 -589934592，真的回绕成负数；80 亿同时超过 unsigned int 上限 4294967295， 所以检查器里「unsigned int 还是装不下」那条指正依旧成立。讲义、三步卡、脚手架 注释、检查指正与提示文案统一成同一组数字。 新增 tests/s5-03-accumulate-overflow-example.test.ts 把例子绑到 int32 回绕计算： 题面写的输出必须等于回绕结果且为负、手算总和必须是项数乘以每项、必须同时超过 int 与 unsigned int 上限，旧说法（10^10 量级、乘系数）不得再出现。证据记进 docs/review/question-verification.json 的 findings。 验证：bun test 783 pass / 0 fail（含新增 6 项）；CSP_E2E_LESSONS=s5-03 跑 s2-typed-cdp.ts 17/17、flow-cdp.ts 19/19、keyboard-cdp.ts 8/8，真机浏览器里 检查器指正与渐进提示已是新的 80 亿量级说法。 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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问题（可证伪的事实错误）
lessons/s5-03任务 2 的改型卡用一个累加例子讲int回绕，题面写「手算总和是正数，程序却输出负数」。两个历史版本的数字本机g++实测都回绕成正数：int输出孩子照着跑看不到负数，会得出「没有溢出」的相反结论——正好把本课要防的坑教反。同一个说法还散在三步对照卡和脚手架注释里。
修法
改成
n=100000、每项80000：手算总和8000000000（80 亿，正数），int实测输出-589934592，真的回绕成负数；80 亿同时超过unsigned int上限4294967295，所以检查器里「unsigned int 还是装不下」那条指正依旧成立。讲义、三步卡、脚手架注释、检查指正、渐进提示统一成同一组数字。TDD
先写
tests/s5-03-accumulate-overflow-example.test.ts并确认失败（当时页面里找不到可机检的场景行），再改页面到绿。护栏把例子绑到 int32 回绕计算：INT32_MAX与UINT32_MAX10^10量级、乘系数）不得再出现改数字而不重新验证，这个测试会红。
验证证据
bun test：783 pass / 0 fail（64 文件；修复前 777，新增 6 项）CSP_E2E_LESSONS=s5-03）：s2-typed-cdp.ts17/17、flow-cdp.ts19/19、keyboard-cdp.ts8/8；输出里检查器指正与提示已是新的 80 亿量级说法docs/review/question-verification.json的findings，并在tests/e2e/s5-03-manual-checklist.md补了本轮复验行边界
只改事实错误与护栏，未动课程结构、题库、
lesson.json、验收矩阵；不涉及在线判题、账号、视频资产或 Node 工具链。