fix(modules): search system data dirs for built-in modules (#338)

packaged installs put the binary in /usr/bin and the modules under
/usr/share/sif/modules, so the loader found zero built-in modules with no
error. add the freedesktop data dirs ($XDG_DATA_DIRS, default
/usr/local/share and /usr/share) as fallback search paths after the existing
executable-relative and working-dir locations, first-existing wins. the new
IsDir check also makes resolution stricter: a plain file named modules no
longer counts as the directory.

internal/modules/loader.go

@@ -50,17 +50,7 @@ func NewLoader() (*Loader, error) {
 		return nil, fmt.Errorf("get home dir: %w", err)
 	}
 
-	// Find built-in modules relative to executable
-	execPath, err := os.Executable()
-	if err != nil {
-		execPath = "."
-	}
-	builtinDir := filepath.Join(filepath.Dir(execPath), "modules")
-
-	// Also check current working directory for development
-	if _, err := os.Stat(builtinDir); os.IsNotExist(err) {
-		builtinDir = "modules"
-	}
+	builtinDir := resolveBuiltinDir()
 
 	// User modules directory based on OS
 	var userDir string
@@ -78,6 +68,53 @@ func NewLoader() (*Loader, error) {
 	}, nil
 }
 
+// resolveBuiltinDir picks the built-in modules directory: the first existing
+// candidate, or the working-directory default when none are present (LoadAll
+// then logs "no built-in modules found" as before).
+func resolveBuiltinDir() string {
+	if dir := firstExistingDir(builtinDirCandidates()); dir != "" {
+		return dir
+	}
+	return "modules"
+}
+
+// builtinDirCandidates lists the directories to probe for built-in modules,
+// most specific first: next to the executable, the working directory (for
+// development), then the freedesktop system data dirs so packaged installs
+// (modules under /usr/share/sif) are found too.
+func builtinDirCandidates() []string {
+	candidates := make([]string, 0, 4)
+
+	if execPath, err := os.Executable(); err == nil {
+		candidates = append(candidates, filepath.Join(filepath.Dir(execPath), "modules"))
+	}
+	candidates = append(candidates, "modules")
+
+	for _, dir := range dataDirs() {
+		candidates = append(candidates, filepath.Join(dir, "sif", "modules"))
+	}
+
+	return candidates
+}
+
+// dataDirs returns the freedesktop base data directories, honoring
+// $XDG_DATA_DIRS and falling back to the spec default when it is unset.
+func dataDirs() []string {
+	if env := os.Getenv("XDG_DATA_DIRS"); env != "" {
+		return filepath.SplitList(env)
+	}
+	return []string{"/usr/local/share", "/usr/share"}
+}
+
+func firstExistingDir(candidates []string) string {
+	for _, dir := range candidates {
+		if info, err := os.Stat(dir); err == nil && info.IsDir() {
+			return dir
+		}
+	}
+	return ""
+}
+
 // LoadAll discovers and loads all modules from both built-in
 // and user directories.
 func (l *Loader) LoadAll() error {

internal/modules/loader_test.go

@@ -267,3 +267,60 @@ func TestLoaderScriptStubNoop(t *testing.T) {
 		t.Errorf("loadScript stub returned error: %v", err)
 	}
 }
+
+func TestDataDirs(t *testing.T) {
+	t.Setenv("XDG_DATA_DIRS", "/opt/a"+string(os.PathListSeparator)+"/opt/b")
+	if got := dataDirs(); len(got) != 2 || got[0] != "/opt/a" || got[1] != "/opt/b" {
+		t.Errorf("dataDirs() with XDG set = %v, want [/opt/a /opt/b]", got)
+	}
+
+	t.Setenv("XDG_DATA_DIRS", "")
+	if got := dataDirs(); len(got) != 2 || got[0] != "/usr/local/share" || got[1] != "/usr/share" {
+		t.Errorf("dataDirs() default = %v, want [/usr/local/share /usr/share]", got)
+	}
+}
+
+func TestFirstExistingDir(t *testing.T) {
+	tmp := t.TempDir()
+	realDir := filepath.Join(tmp, "real")
+	if err := os.Mkdir(realDir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	file := writeModule(t, tmp, "afile", "x")
+
+	if got := firstExistingDir([]string{filepath.Join(tmp, "missing"), file, realDir}); got != realDir {
+		t.Errorf("firstExistingDir = %q, want %q (a file must be skipped)", got, realDir)
+	}
+	if got := firstExistingDir([]string{filepath.Join(tmp, "nope")}); got != "" {
+		t.Errorf("firstExistingDir with no match = %q, want empty", got)
+	}
+}
+
+// TestBuiltinDirCandidatesIncludesDataDirs is the FHS regression: packaged
+// installs keep modules under <data-dir>/sif/modules, so that path must be a
+// candidate, ordered after the executable-relative and working-dir paths.
+func TestBuiltinDirCandidatesIncludesDataDirs(t *testing.T) {
+	t.Setenv("XDG_DATA_DIRS", "/usr/share")
+	candidates := builtinDirCandidates()
+
+	want := "/usr/share/sif/modules"
+	if candidates[len(candidates)-1] != want {
+		t.Errorf("candidates %v missing trailing data-dir path %q", candidates, want)
+	}
+}
+
+// TestResolveBuiltinDirFindsPackagedModules is the resolveBuiltinDir-level
+// counterpart to TestBuiltinDirCandidatesIncludesDataDirs above.
+func TestResolveBuiltinDirFindsPackagedModules(t *testing.T) {
+	tmp := t.TempDir()
+	pkg := filepath.Join(tmp, "sif", "modules")
+	if err := os.MkdirAll(pkg, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("XDG_DATA_DIRS", tmp)
+	t.Chdir(t.TempDir()) // scratch cwd so the "modules" candidate does not exist
+
+	if got := resolveBuiltinDir(); got != pkg {
+		t.Errorf("resolveBuiltinDir = %q, want packaged %q", got, pkg)
+	}
+}