fix(schedule): four defects found reviewing bmi_schedule, all reachable

深度 review 这条分支时发现的,四条都能走到,其中两条会让构建**无输出地卡死**。

## 1. 默认配置下 detach-codegen 的并发**完全没有上限**

模块自己的注释写着「真正的并发由信号量限制,永远不是 -j」——
而 `resolve_jobs` 在用户没给 `--jobs` 时返回 0(「不说话,用后端默认」),
于是 `compilerCap = 0` → `acquire_token` 直接返回空 → **信号量被禁用**,
生成的图里是 `sched_cap = 0`。

detach 之后编译器一发布 BMI 就交还 ninja 槽位,所以 ninja 的 -j 已经不是上限了。
结果:**ninja 有多快就起多少个编译器,没有任何东西在数**,而单个模块编译峰值接近
1GB。这是模块自己列的 hazard 2,而它在**默认路径上根本没生效**。

修法:`decide()` 保持纯函数,新增 `autoJobs` 参数由调用方(prepare)算好传入;
detach 分支在没有显式 job 数时回落到这台机器的推荐值。实测 `sched_cap` 从 0
变成 24。两者都没有时,**reason 里直接说「bounded by nothing」**。

## 2. `.mcpp-sched` 的令牌**没有任何人回收**

令牌由持有它的 supervisor 删除。supervisor 没跑到清理就死掉(对构建按 Ctrl-C、
OOM killer、重启)就会留下目录,而没有任何代码删它 —— 每发生一次就**永久**少掉
一个槽位,攒够 cap 个之后下一次构建等一个永远不会被释放的令牌,**没有输出、
没有诊断**。对构建按 Ctrl-C 是再平常不过的操作。

⚠️ 我第一次把回收放在了 prepare 里(schedule 决策旁边),**e2e 证明它根本没跑**:
增量构建走工程级 fast path,直接重放 build.ninja,不重新推导 plan。回收必须放在
**每次构建都会经过**的地方,也就是 execute 里 spawn ninja 之前。

## 3. phase 1 等 BMI 是 `for(;;)`,没有上限

phase 2 有明确的上限,注释还写着「无上限的等待比失败更糟,因为没有任何东西告诉你
该看哪」—— 而**等同一个 supervisor 的 phase 1 没有**。supervisor 被 OOM 杀掉就
永远等下去。补上 10s「supervisor 没起来」宽限 + 2h 硬上限,两者都会恢复旧 BMI。
`acquire_token` 同样加了上限:超时后**不带 cap 继续**(信号量限的是内存压力,
不是正确性,失败的构建比慢的构建更糟),并说清楚该删哪个目录。

## 4. supervisor 有一条退出路径不写 `.rc`

命令文件读不出来时 `cmd_bmi_supervise` 直接 return 2,而 phase 1/2 都在等那个文件
—— 和「supervisor 从没启动」无法区分,两边只能等到超时。现在这种情况也写 rc。

## 测试

`tests/e2e/234_bmi_schedule_on.sh`(**这个特性此前没有任何 e2e**,只有 `decide()`
的单测,测的是**表**不是机制)。它种下满额的陈旧令牌,要求下一次构建照样跑完。
这个测试**在修好之前红了两次**:第一次真的卡死 600s,第二次暴露了我把回收放错了
地方 —— 不是空转的。

单测补了两条:零 cap 必须**在 reason 里说自己没有上限**;detach 在没有 job 数时
必须回落到宿主推荐值,而显式 `--jobs` 仍然优先。

本地:83 个单测全过,234 过。

Author: speak-agent

bench/results/standard-20260814-linux-x86_64.superseded-225002/gcc-fixture.log

@@ -0,0 +1,153 @@
+payload: payload:gcc → /home/speak/.mcpp/registry/data/xpkgs/xim-x-gcc/16.1.0/bin/g++
+run id : 4e58a816 (fresh)
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+fixture: 20 units, fanin 3, weight 4
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