a .md file starting with _ is a fragment, not a bundle

Found by serving the include evidence: `tools.md` beside a bundle's index was
itself scanned as a bundle, so a file meant only to be included took a URL of its
own, appeared in its section's listing, and turned the including bundle into a
one-member series. The real binary showed the phantom series nav; no test would
have, because every fixture happened to name its partials differently.

The rule mirrors the one directories already have, `_index` excepted since that
names its directory. `{{< include file="_tools.md" >}}` is now the shape to write.

internal/content/content_test.go

@@ -4,6 +4,7 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"sort"
 	"testing"
 	"testing/fstest"
 )
@@ -173,3 +174,32 @@ func TestTagSlugPreservesScriptAndFoldsCase(t *testing.T) {
 		}
 	}
 }
+
+func TestAnUnderscoreFileIsAPartialNotABundle(t *testing.T) {
+	// A file meant only to be included must not also be a bundle: it would take a URL, show up in its
+	// section's listing, and make its bundle look like a one-member series (content-model.md).
+	fsys := fstest.MapFS{
+		"content/pages/about/index.md":     {Data: []byte("---\ntitle: About\n---\nx\n")},
+		"content/pages/about/_tools.md":    {Data: []byte("A fragment.\n")},
+		"content/pages/about/_notes.bn.md": {Data: []byte("একটি অংশ।\n")},
+		"content/pages/_index.md":          {Data: []byte("---\ntitle: Pages\n---\n")},
+	}
+	bundles, err := Scan(fsys)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var keys []string
+	for _, b := range bundles {
+		keys = append(keys, b.Key)
+	}
+	sort.Strings(keys)
+	want := []string{"pages", "pages/about"}
+	if len(keys) != len(want) {
+		t.Fatalf("scanned %v, want %v — _index still names its directory", keys, want)
+	}
+	for i := range want {
+		if keys[i] != want[i] {
+			t.Fatalf("scanned %v, want %v", keys, want)
+		}
+	}
+}