fix(toolchain): per-file C++ force for cl, so object inputs stay objects

cl.exe rejected the std and mcpp module objects with
"C2018: character 'U+10' is not permitted here" — it was compiling them as
C++ source. `/TP` is not the counterpart of `-x c++`: GNU's is positional and
lasts until `-x none`, cl's applies to EVERY input on the line, so the
objects that follow are fed to the C++ frontend.

The dialect now carries the per-FILE form (`/Tp<file>`) alongside the
positional one, and the build.mcpp compile uses whichever the driver has.
That is the same structural difference the row already records for libFlag
(prefix vs suffix): the two drivers are not spelling the same concept
differently, they have different concepts.

Author: Sunrisepeak

.agents/docs/2026-08-02-host-compile-implementation-plan.md

@@ -219,6 +219,16 @@ diff /tmp/stdcmd-base.txt /tmp/stdcmd-after.txt                            # 必
 
 ---
 
+## 实施记录(只有 CI 能发现的)
+
+| 发现 | 教训 |
+|---|---|
+| **扩大 `build_program.cppm` 的匿名 ns 同样触发 clang 误编译** | 把 `build_mcpp_module` 在原地改写加大 → macOS 全部 build.mcpp e2e 段错误,和 PR#332 一模一样。约束不是「别加新函数」,是**「别再往那个 ns 加代码」**。修法=整块搬到 `src/build/hostprogram.cppm` |
+| **「扫缓存比对字节等价」是假验证** | std 缓存共享且累积,两次快照都含**陈旧条目**,diff 恒为空 —— 我因此放过了一次真实的字符串改动(`-stdlib=libc++` 位置)。**把主张写成测试**:用合成的 `ClangDriverModel`/`ToolchainLinkModel` 直接断言渲染出的字面串 |
+| **`-stdlib=libc++` 的位置是兼容面** | 它进 `std_build_commands` → 进 metadata → **决定 std 缓存目录名**。挪一个 flag = 让每个用户的 std BMI 全量失效 |
+| **「信任 cfg」不等于「什么都不发」** | 生产者在 trust-cfg 分支提前 `return`,把 deployment target 也跳过了;而 macOS 上 build.mcpp 走的正是这条分支 → std BMI 配置不匹配。旧的手写实现把它放在**最前、无条件**,注释还专门写了 "FIRST and unconditionally" —— 重构时要读懂那句话为什么在 |
+| **删掉能力门 = 死代码变活代码** | `-x none` 常年无条件发出,只因 MSVC 到不了那里才无害。门一删,cl 立刻 `D9002: ignoring unknown option '-x'`。**删门时要把门后所有「反正到不了」的分支重新审一遍** |
+
 ## Self-Review
 
 **设计覆盖**:§4.1 token 生产者 → Task 2/3;§4.2 三种渲染 → Task 4;

src/build/build_program.cppm

@@ -663,8 +663,15 @@ std::expected<void, std::string> run_build_program(
     for (auto& sf : stdFlags)    compileArgv.push_back(sf);
     // The `.mcpp` extension is unknown to every driver, so without this the
     // file is handed to the linker as a linker script.
-    for (auto f : dial.forceCxxLangArgv) compileArgv.emplace_back(f);
-    compileArgv.push_back(src.string());
+    // Per-file where the driver has that form (cl's /Tp), positional
+    // otherwise. Object files follow on this same command line, and cl's
+    // global /TP would compile them as C++ source.
+    if (!dial.perFileCxxPrefix.empty()) {
+        compileArgv.push_back(std::string(dial.perFileCxxPrefix) + src.string());
+    } else {
+        for (auto f : dial.forceCxxLangArgv) compileArgv.emplace_back(f);
+        compileArgv.push_back(src.string());
+    }
     if (usesModule || !stdObjects.empty()) {
         // Link the module objects. GNU drivers need the input language reset
         // first, or the .o that follows `-x c++` is handed to the frontend as

src/toolchain/dialect.cppm

@@ -54,6 +54,14 @@ struct CommandDialect {
     // better off without (see the note on `alwaysFlagsArgv`).
     std::string_view forceCxxLang;     // "-x c++" | "/TP"
     std::span<const std::string_view> forceCxxLangArgv;
+    // Per-FILE language force, for a command line that also carries object
+    // files. The two drivers differ structurally, not just in spelling: GNU's
+    // `-x c++` is positional and stays in effect until `-x none`, while
+    // cl.exe's `/TP` applies to EVERY input — so an object listed after it is
+    // fed to the C++ frontend and dies with C2018. cl's per-file form is
+    // `/Tp<file>`; GNU has none, and uses the positional pair plus a reset.
+    // Empty means "no per-file form — use forceCxxLangArgv and reset after".
+    std::string_view perFileCxxPrefix; // ""       | "/Tp"
     // Static CRT / runtime. On MSVC this is a compile-time CRT model, not a
     // link mode — there is no /MT equivalent of `-static` for the whole image.
     std::string_view staticRuntime;    // "-static"| "/MT"
@@ -127,6 +135,7 @@ constexpr CommandDialect kGnuDialect{
     .libSearchPrefix = "-L",
     .forceCxxLang    = "-x c++",
     .forceCxxLangArgv = kGnuForceCxxArgv,
+    .perFileCxxPrefix = "",
     .staticRuntime   = "-static",
     .outputExePrefix = "-o ",
     .objExt          = ".o",
@@ -154,6 +163,7 @@ constexpr CommandDialect kMsvcDialect{
     .libSearchPrefix = "/LIBPATH:",
     .forceCxxLang    = "/TP",
     .forceCxxLangArgv = kMsvcForceCxxArgv,
+    .perFileCxxPrefix = "/Tp",
     .staticRuntime   = "/MT",
     .outputExePrefix = "/Fe:",
     .objExt          = ".obj",