fix(cmd): usage exit codes, asm output file and cross-arch ground truth

Assisted-by: GLM 5.3

cmd/gasm/audit.go

@@ -63,7 +63,7 @@ the encodability backlog by frequency rather than by table order.
 	archName := "amd64"
 	switch n := len(fs.Args()); {
 	case n > 1:
-		return fmt.Errorf("audit-instructions takes at most one architecture argument")
+		return &usageError{fmt.Errorf("audit-instructions takes at most one architecture argument")}
 	case n == 1:
 		archName = strings.ToLower(fs.Arg(0))
 	}
@@ -136,7 +136,7 @@ func auditArch(name string) (arch.Arch, error) {
 	case "loong64", "loong":
 		return arch.LOONG64, nil
 	}
-	return arch.Unknown, fmt.Errorf("unknown architecture %q: want amd64, arm64, riscv64 or loong64", name)
+	return arch.Unknown, &usageError{fmt.Errorf("unknown architecture %q: want amd64, arm64, riscv64 or loong64", name)}
 }
 
 // goarchName maps an arch identifier onto its GOARCH spelling.
@@ -217,6 +217,13 @@ func probeGoAsm(goarch string, names []string) (map[string]bool, error) {
 	cmd := exec.Command(asmBin, "-p", "probe", "-o", filepath.Join(dir, "probe.o"), probePath)
 	cmd.Env = append(os.Environ(), "GOARCH="+goarch, "GOOS="+runtime.GOOS)
 	out, _ := cmd.CombinedOutput()
+	// The expected failure mode is a non-zero exit with compiler diagnostics
+	// on stdout; empty output means the probe broke at the exec level (a
+	// killed child, a tool that would not start), and seeding every name as
+	// recognized on that silence would fake a clean audit.
+	if len(out) == 0 {
+		return nil, fmt.Errorf("go tool asm probe for GOARCH=%s produced no output", goarch)
+	}
 
 	result := map[string]bool{}
 	for _, name := range names {
@@ -344,7 +351,7 @@ func (t *corpusTally) fail(path, reason string) {
 // cmdAuditCorpus implements audit-instructions --corpus.
 func cmdAuditCorpus(args []string) error {
 	if len(args) > 1 {
-		return fmt.Errorf("audit-instructions --corpus takes at most one directory argument")
+		return &usageError{fmt.Errorf("audit-instructions --corpus takes at most one directory argument")}
 	}
 	root := ""
 	if len(args) == 1 {