fix(pack): print the compiler's output when the packer's build fails

Windows CI reported

    error: build failed
    FAIL: shared pack failed off ELF

and nothing else, because `mcpp pack` printed `BuildError::message` and dropped
`BuildError::diagnosticOutput` — the field that carries what the compiler
actually said. `mcpp build` has printed it all along; the two pack pipelines
were the only callers that did not.

That is a defect on its own terms, not just an inconvenience for this
investigation: a packaging failure is exactly when a maintainer has least context,
and three words is not a report. Both pipelines now print it.

Author: speak-agent

src/pack/library_pipeline.cppm

@@ -14,6 +14,13 @@
 //
 // Design: .agents/docs/2026-08-17-library-distribution-design.md §2.
 
+module;
+// `stderr` / `fputs`: the packer prints the compiler's own diagnostics, and a
+// build failure that arrives as three words is a build failure nobody can act
+// on. This global module fragment was removed once as an unused `<cstdio>`;
+// it has a use now.
+#include <cstdio>
+
 export module mcpp.pack.library_pipeline;
 
 import std;
@@ -158,6 +165,14 @@ export int build_and_pack_library(const std::string& targetName,
         auto be = mcpp::build::make_ninja_backend();
         mcpp::build::BuildOptions bo;
         if (auto br = be->build(ctx->plan, bo); !br) {
+            // The compiler's own output, not just "build failed". `mcpp build`
+            // has always printed this; `mcpp pack` dropped it, so a failure
+            // inside the packer's build arrived as three words and CI logs had
+            // nothing to go on.
+            if (!br.error().diagnosticOutput.empty()) {
+                std::fputs(br.error().diagnosticOutput.c_str(), stderr);
+                if (br.error().diagnosticOutput.back() != '\n') std::fputs("\n", stderr);
+            }
             mcpp::ui::error(br.error().message);
             return 1;
         }

src/pack/pipeline.cppm

@@ -86,6 +86,12 @@ export int build_and_pack(Options opts, bool modeFromUser,
     mcpp::build::BuildOptions bo;
     auto br = be->build(ctx->plan, bo);
     if (!br) {
+        // The compiler's own output, not just "build failed" — same reason as
+        // in the library pipeline.
+        if (!br.error().diagnosticOutput.empty()) {
+            std::fputs(br.error().diagnosticOutput.c_str(), stderr);
+            if (br.error().diagnosticOutput.back() != '\n') std::fputs("\n", stderr);
+        }
         mcpp::ui::error(br.error().message);
         return 1;
     }