fix: decide a build step by its exit code, not by whether it printed

RunCommand merged stderr into stdout, dropped pclose's status and returned
the text; every compile, link and archive site then read "printed something"
as "failed". A warning is printing something, so a translation unit that
warned failed the build — but only on the run that actually recompiled it,
since warnings aren't re-emitted for an object that's already up to date.
The same unchanged source therefore passed or failed depending on the state
of the build tree: flaky-looking tests locally, and a cold CI checkout
surfacing every latent warning in a project at once as unrelated failures.

RunCommand is gone, replaced by RunBuildCommand: it goes through
RunCommandChecked, returns "" when the command exited 0 (so every caller's
`if (!result.empty())` error path is unchanged) and hands any warnings to
the new Progress::Diagnostic instead of to the error path. That also closes
the quiet half of the bug — a compiler killed by the OOM killer prints
nothing, so it used to read as success and leave the build carrying on with
a missing object; it now reports the signal that killed it.

Warnings are now shown rather than swallowed, which they weren't in either
direction before: invisible on an incremental build, fatal on a cold one.
Failing on them stays a project's choice, via -Werror in compileFlags.

implementations/Crafter.Build-Implementation.cpp

@@ -52,7 +52,7 @@ namespace Crafter {
         }
 
         // -MD leaves <name>_impl.o.d beside the object for Check to read.
-        std::string result = RunCommand(std::format("{0} {1}.cpp -c -MD -MF {2}_impl.o.d -o {2}_impl.o", clang, path.string(), (buildDir/path.filename()).string()));
+        std::string result = RunBuildCommand(std::format("{0} {1}.cpp -c -MD -MF {2}_impl.o.d -o {2}_impl.o", clang, path.string(), (buildDir/path.filename()).string()));
 
         bool expected = false;
         if(!result.empty() && buildCancelled.compare_exchange_strong(expected, true)) {