resolve sequences from the directory tree

A bundle nested under another bundle is a member of that series (ADR-0033), so
`Site.Sequence` walks up to the nearest bundle ancestor and back down to its
members: ordered by `order` where set, then by name. Members resolve through the
language fallback, so a chapter with no Bengali variant still holds its place in
Bengali reading order instead of breaking prev/next.

One `.Sequence` field carries both shapes a theme needs. A landing page renders
`.Members` as an archive; a chapter renders `.Prev`/`.Next`, which are pointers
into `.Members` so `{{with}}` yields nothing at the ends. `Index == 0` is what
tells the two apart.

`Query` was deliberately not extended. A series ascends where `Run` descends, and
an order knob on `Query` is the config knob rule 6 bans; instead `Site.keys()`
came out so both iterate the index one way, deleting `Run`'s own dedupe map.

`draft` is not honoured: no bundle carries the field and nothing else excludes
drafts, so entry 19 adds it in both places at once. Recorded in content-model.md
rather than left implied.

state.md also corrects six inventory rows that had drifted before this change —
three LOC figures, the test total, `go.mod`, and two lines that were flatly wrong
("Dependencies: none", "goldmark is not yet imported"). The coupling gate proves
state.md changed with the code; it cannot prove the numbers are right.

internal/content/site.go

@@ -119,17 +119,11 @@ type Query struct {
 // Run applies q, newest first, with undated bundles after dated ones and ties broken by key so the same
 // query always answers in the same order.
 func (s *Site) Run(q Query) []Bundle {
-	seen := map[string]bool{}
 	var out []Bundle
-	for kl := range s.byKeyLang {
-		key, _, found := strings.Cut(kl, "\x00")
-		if !found || seen[key] {
-			continue
-		}
+	for _, key := range s.keys() {
 		if q.Section != "" && !strings.HasPrefix(key, q.Section+"/") {
 			continue
 		}
-		seen[key] = true
 		b, _, ok := s.Lookup(key, q.Lang)
 		if !ok || !b.hasTag(q.Tag) {
 			continue
@@ -148,6 +142,107 @@ func (s *Site) Run(q Query) []Bundle {
 	return out
 }
 
+// keys lists every bundle key once, sorted, so nothing iterating the index depends on map order.
+func (s *Site) keys() []string {
+	seen := make(map[string]bool, len(s.byKeyLang))
+	out := make([]string, 0, len(s.byKeyLang))
+	for kl := range s.byKeyLang {
+		key, _, found := strings.Cut(kl, "\x00")
+		if !found || seen[key] {
+			continue
+		}
+		seen[key] = true
+		out = append(out, key)
+	}
+	sort.Strings(out)
+	return out
+}
+
+// Sequence is the reading order a bundle participates in: a series and its members.
+type Sequence struct {
+	// Series is the landing bundle — comics/the-long-monsoon for its chapters.
+	Series Bundle
+	// Members are the series' bundles in reading order.
+	Members []Bundle
+	// Index is the 1-based position of the bundle asked about, and zero when that bundle is the landing
+	// page itself.
+	Index int
+}
+
+// Sequence resolves the series a bundle participates in, reporting false when it is in none.
+//
+// Membership is structural (ADR-0033): a bundle with bundles nested under it is a series landing page and
+// they are its members, and a bundle nested under another is a member of that one. A landing page inside
+// another series reports its own members rather than its siblings, since the deeper series is what the
+// page is about.
+//
+// Members resolve through the language fallback chain, so a chapter missing in this language still holds
+// its place in the reading order rather than breaking prev/next (ADR-0009). `draft` is not honoured
+// because no bundle carries it yet (content-model.md).
+func (s *Site) Sequence(key, lang string) (Sequence, bool) {
+	if members := s.members(key, lang); len(members) > 0 {
+		// key names a bundle: members are the bundles nested under it.
+		landing, _, _ := s.Lookup(key, lang)
+		return Sequence{Series: landing, Members: members}, true
+	}
+	series, nested := s.parent(key)
+	if !nested {
+		return Sequence{}, false
+	}
+	landing, _, _ := s.Lookup(series, lang) // parent only names bundles
+	seq := Sequence{Series: landing, Members: s.members(series, lang)}
+	for i, m := range seq.Members {
+		if m.Key == key {
+			seq.Index = i + 1
+		}
+	}
+	return seq, true
+}
+
+// parent is the nearest ancestor of key that is itself a bundle, in any language.
+func (s *Site) parent(key string) (string, bool) {
+	for i := strings.LastIndex(key, "/"); i > 0; i = strings.LastIndex(key[:i], "/") {
+		if ancestor := key[:i]; s.has(ancestor) {
+			return ancestor, true
+		}
+	}
+	return "", false
+}
+
+// has reports whether a key exists in any language.
+func (s *Site) has(key string) bool {
+	_, _, ok := s.Lookup(key, "")
+	return ok
+}
+
+// members are the bundles whose nearest bundle ancestor is series, in reading order.
+//
+// order ascending where it is set, then by key: a member without order sorts after every member carrying
+// one, mirroring how an undated bundle sorts after dated ones (ADR-0033).
+func (s *Site) members(series, lang string) []Bundle {
+	var out []Bundle
+	for _, key := range s.keys() {
+		if p, nested := s.parent(key); !nested || p != series {
+			continue
+		}
+		if b, _, ok := s.Lookup(key, lang); ok {
+			out = append(out, b)
+		}
+	}
+	sort.SliceStable(out, func(i, j int) bool {
+		a, b := out[i], out[j]
+		switch {
+		case (a.Order == 0) != (b.Order == 0):
+			return b.Order == 0
+		case a.Order != b.Order:
+			return a.Order < b.Order
+		default:
+			return a.Key < b.Key
+		}
+	})
+	return out
+}
+
 // hasTag reports whether the bundle carries a tag slug. An empty slug matches everything.
 func (b Bundle) hasTag(slug string) bool {
 	if slug == "" {