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.

docs/content-model.md

@@ -59,8 +59,10 @@ overrides the defaults the binary embeds, so a bare root still renders.
   section or series landing page) rather than a plain container.
 - Local assets sit beside the body, referenced relatively. Assets never live in frontmatter. Moving a
   bundle moves its assets — the entire point of bundles.
-- A directory starting with `_` other than `_index` is ignored. `draft: true` is excluded from queries
-  and feeds but reachable at its own URL in dev.
+- A directory starting with `_` other than `_index` is ignored, and so is a **file**: `_tools.md` is a
+  fragment, not a bundle. That is how a file meant only to be included avoids taking a URL of its own,
+  appearing in its section's listing, and turning its bundle into a one-member series.
+- `draft: true` is excluded from queries and feeds but reachable at its own URL in dev.
 
 ## Frontmatter
 
@@ -298,6 +300,9 @@ all of them consequences of ADR-0038:
 
 A `gallery` inside an included file still resolves against the same bundle.
 
+Name an included file with a leading underscore — `{{< include file="_tools.md" >}}` — or it is a bundle too,
+with its own URL.
+
 Sharing one fragment between bundles is deliberately not possible yet: it needs somewhere to keep shared
 parts, which is a decision about the disk contract rather than a missing feature. Transclusion of another
 bundle's *body* is Arc 4 and needs a cycle guard of its own.

docs/state.md

@@ -9,7 +9,7 @@ If this file disagrees with the code, the code is right and this file is a bug.
 |---|---|---|
 | `go.mod` | module `khosra`; `goldmark`, `x/text`, `yaml.v3` direct | 10 |
 | `internal/content/doc.go` | package comment | 5 |
-| `internal/content/content.go` | bundles: `os.Root` open, walk, frontmatter split, key/lang derivation, NFC, tag slugs, permalink building | 352 |
+| `internal/content/content.go` | bundles: `os.Root` open, walk, frontmatter split, key/lang derivation, NFC, tag slugs, partial files, permalink building | 368 |
 | `internal/content/site.go` | the indexed site: lookup with language fallback, aliases, `Query` and `Run`, sections, `Sequence` | 286 |
 | `internal/render/render.go` | goldmark with the typographer, per-kind template sets with site override, the `Partial`/`Origin` seams features render and resolve through, `Page`/`List`/`Sequence`/`head` | 386 |
 | `internal/render/chrome.go` | the engine's own words: phrase table, month names, digits, and the `t`/`num`/`day` template funcs (ADR-0034) | 105 |
@@ -19,7 +19,7 @@ If this file disagrees with the code, the code is right and this file is a bug.
 | `internal/web/resolve.go` | URL → (key, lang, page, tag) or a canonical redirect: language prefix, `/en/…` fork guard, pagination, tags, trailing slash | 112 |
 | `internal/web/web.go` | handler: resolve, look up with fallback, section and tag listings, sequence, `/static/` (misses and refusals alike answer 404), degrade on failure | 152 |
 | `cmd/khosra/main.go` | flags, wiring, startup — the only place things are assembled | 53 |
-| `*_test.go` | table-driven, one file per source file; symlink escape (content and static), canonical paths, language fallback, aliases, pagination, tags, sequences, chrome, typography, shortcode escaping, galleries, includes, 404 | 1437 |
+| `*_test.go` | table-driven, one file per source file; symlink escape (content and static), canonical paths, language fallback, aliases, pagination, tags, sequences, chrome, typography, shortcode escaping, galleries, includes, partials, widows, 404 | 1562 |
 
 Serves a bundle at `/{section}/{slug}/`, a paginated listing per section, tag listings global and
 section-narrowed, sequence navigation and a series archive on any nested bundle, and `static/` verbatim.

internal/content/content.go

@@ -84,6 +84,8 @@ func Scan(fsys fs.FS) ([]Bundle, error) {
 			return nil
 		case !strings.HasSuffix(p, ".md"):
 			return nil
+		case isPartial(path.Base(p)):
+			return nil
 		}
 		data, err := fs.ReadFile(fsys, p)
 		if err != nil {
@@ -258,6 +260,20 @@ func isLangTag(s string) bool {
 	return true
 }
 
+// isPartial reports whether a filename is a fragment rather than a bundle of its own.
+//
+// An underscore prefix, the same mark a directory already uses, with `_index` excepted because that names
+// the directory it sits in. Without this a file meant only to be included would also be a bundle: it would
+// take a URL, appear in its section's listing, and turn its bundle into a one-member series
+// (content-model.md).
+func isPartial(base string) bool {
+	name := strings.TrimSuffix(base, ".md")
+	if i := strings.LastIndex(name, "."); i > 0 && isLangTag(name[i+1:]) {
+		name = name[:i]
+	}
+	return strings.HasPrefix(name, "_") && name != "_index"
+}
+
 // skipDir reports whether a directory is not content: hidden, or underscore-prefixed.
 func skipDir(base string) bool {
 	return strings.HasPrefix(base, ".") && base != "." || strings.HasPrefix(base, "_")

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)
+		}
+	}
+}