fix(main): move the stdout buffering call into mcpp.ui

`module;` opens a global module fragment, which is only legal in a module unit.
main.cpp is not one, so the fragment introduced for <cstdio> made it ill-formed:
GCC accepted it silently, Clang rejected it outright ("missing 'module'
declaration at end of global module fragment") and every macOS job — where the
toolchain is Clang — failed to build.

The policy belongs in mcpp.ui anyway: that module already owns "when does output
become visible", already has a legitimate global module fragment for <cstdio>,
and already exports flush() for the Windows half of the same guarantee.

Verified by building mcpp with llvm@22.1.8 locally, not just gcc — the class of
bug this was is invisible to a single-compiler check.

Author: Sunrisepeak

src/main.cpp

@@ -1,11 +1,9 @@
 // mcpp — Modular C++ Package Manager & Build Tool
 // Entry point — delegates to mcpp.cli command dispatch.
 
-module;
-#include <cstdio>      // setvbuf, stdout, _IOLBF
-
 import std;
 import mcpp.cli;
+import mcpp.ui;
 
 int main(int argc, char* argv[]) {
     // Line-buffer stdout even when it is not a TTY.
@@ -21,9 +19,11 @@ int main(int argc, char* argv[]) {
     // mcpp's. When that run was then killed by the job timeout, the whole buffer
     // went with it — a 45-minute step with no attributable output at all.
     //
+    // Lives in mcpp.ui because a non-module TU may not open a global module
+    // fragment for <cstdio> (Clang rejects `module;` here; GCC accepted it).
     // ui::flush() covers the same ground for the ui layer on Windows, where
     // MSVCRT silently treats _IOLBF as _IOFBF.
-    std::setvbuf(stdout, nullptr, _IOLBF, 0);
+    mcpp::ui::set_line_buffered();
 
     int rc;
     try {

src/ui.cppm

@@ -81,6 +81,13 @@ void plain(std::string_view message);
 // when its last lines matter most.
 void flush();
 
+// Make stdout line-buffered. Call once, before any output. See main() for why
+// this is not left to the libc default: the default block size is a different
+// number on every platform (musl 1024, Apple libc st_blksize = 65536 on a pipe,
+// MSVCRT 4096), so identical output becomes visible at wildly different times —
+// and not at all if the process is killed before its buffer fills.
+void set_line_buffered();
+
 // --- progress bar (single-line, \r-rewritten) ---
 class ProgressBar {
 public:
@@ -237,6 +244,8 @@ bool is_quiet()        { return g_quiet; }
 
 void flush() { std::fflush(stdout); }
 
+void set_line_buffered() { std::setvbuf(stdout, nullptr, _IOLBF, 0); }
+
 void status(std::string_view verb, std::string_view message) {
     if (g_quiet) return;
     init();