Builder diagnostics go to stderr; stdout is the result alone

The logging added a line to stdout, and the genesis path parses the builder's
stdout as JSON — so the first log line broke the parse with "invalid character
'c'", the c from "[clone]". A build that had worked stopped working because of a
print statement.

The installer's runner captures stdout alone (cmd.Output), and the contract was
already stdout=result, stderr=everything else. The fix is to honour it: every
builder diagnostic — the step log, the per-command echo, the module path's own
lines — goes to stderr. Stdout carries only once.go's result JSON.

And a unit test now fails if any fmt.Print to stdout appears in the two builder
command files, except the three that belong there: the result, --version, and
--help. A guard, because this was invisible until a 20-minute run hit it, and the
same class of mistake should fail in milliseconds next time.

Claude-Session: https://claude.ai/code/session_01D6qtiYU3P9jk3pnAXyAFyx

internal/builder/builder.go

@@ -478,16 +478,16 @@ func Command(ctx context.Context, dir, name string, args ...string) (string, err
 	// nothing" and "git clone is waiting on a network that will not answer". Silent on success is
 	// what made an empty workspace unreadable.
 	started := timeNow()
-	fmt.Printf("  $ (%s) %s %s\n", short(filepath.Base(dir)), name, strings.Join(args, " "))
+	fmt.Fprintf(os.Stderr, "  $ (%s) %s %s\n", short(filepath.Base(dir)), name, strings.Join(args, " "))
 	cmd := exec.CommandContext(ctx, name, args...)
 	cmd.Dir = dir
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		fmt.Printf("  ! %s %s failed after %s\n", name, args[0], since(started))
+		fmt.Fprintf(os.Stderr, "  ! %s %s failed after %s\n", name, args[0], since(started))
 		return string(out), fmt.Errorf("%s %s: %w\n%s",
 			name, strings.Join(args, " "), err, strings.TrimSpace(string(out)))
 	}
-	fmt.Printf("  ✓ %s %s (%s)\n", name, firstArg(args), since(started))
+	fmt.Fprintf(os.Stderr, "  ✓ %s %s (%s)\n", name, firstArg(args), since(started))
 	return string(out), nil
 }