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.

interfaces/Crafter.Build-Progress.cppm

@@ -31,6 +31,11 @@ export namespace Crafter::Progress {
     // Verbose-mode command echo. No-op outside Verbose.
     CRAFTER_API void EchoCommand(std::string_view command);
 
+    // Non-fatal output from a build step that succeeded — compiler and linker
+    // warnings. Erases the status line first so the text doesn't land on top of
+    // it, then writes to stderr. No-op on Quiet.
+    CRAFTER_API void Diagnostic(std::string_view text);
+
     // Erase the in-place status line so subsequent stderr writes (errors,
     // banners) don't collide with it. No-op when not in TTY-redraw mode.
     CRAFTER_API void Clear();