Implementation partition in a .cpp file is rebuilt on every build and its BMI is never written #790

An implementation partition (module M:part;) placed in a .cpp file is recompiled on every mcpp build, including builds where nothing changed. The plan declares a BMI for the unit, but the compile step for a .cpp file never writes one, so ninja always sees a missing output.

Minimal project (mcpp 2026.10.8.1, LLVM 23.1.3, --target x86_64-linux-gnu, Linux x86_64):

# mcpp.toml
[package]
name = "repro"
version = "0.1.0"

[toolchain]
default = "llvm@23.1.3"

[targets.repro]
kind = "bin"
main = "src/main.cpp"

// src/repro.cppm
export module repro;

export import :api;

// src/api.cppm
export module repro:api;

export auto answer() -> int;

// src/api_impl.cpp
module repro:api_impl;

import :api;

auto answer() -> int {
    return 42;
}

// src/main.cpp
import repro;
import std;

auto main() -> int {
    std::println("{}", answer());
}

Running mcpp build --target x86_64-linux-gnu --verbose three times: the first build compiles everything and the program prints 42. The second and third builds, with no edits in between, each run the same two steps again, [1/2] compiling src/api_impl.cpp and [2/2] linking.

The generated build.ninja has

build obj/api_impl.o | pcm.cache/repro-api_impl.pcm : cxx_object .../src/api_impl.cpp | obj/api_impl.cpp.ddi.dd

but the compile command is a plain -c .../src/api_impl.cpp -o obj/api_impl.o without -fmodule-output, and pcm.cache/ only ever contains repro.pcm, repro-api.pcm and std.pcm. Because repro-api_impl.pcm never appears, the edge is always dirty.

If any unit imports the partition, the build fails instead. Adding import :api_impl; to src/repro.cppm gives

src/repro.cppm:4:9: fatal error: module 'repro:api_impl' not found

Renaming the file to src/api_impl.cppm makes both cases work, so the problem is limited to the .cpp spelling. I expected mcpp either to build a .cpp file that declares a partition as a module unit and write its BMI, or to refuse it with a message that names the file, the same way an unknown extension is refused. Right now it is accepted silently and the cost only shows up as a rebuild on every run.