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/content.go

@@ -43,6 +43,10 @@ type Bundle struct {
 	// Aliases are paths that must keep resolving to this bundle, each redirecting to its canonical URL
 	// (ADR-0008). Additive only: an alias is a promise never withdrawn.
 	Aliases []string
+	// Order is this bundle's position in the series it is nested under, zero when frontmatter omits it.
+	// The convention is sparse (10, 20, 30), so zero is not a position: an unordered member sorts by name
+	// after every ordered one (ADR-0033).
+	Order int
 	// Body is everything after the frontmatter, unrendered.
 	Body []byte
 	// Extra holds every frontmatter key other than title, exactly as YAML parsed it.
@@ -123,6 +127,8 @@ func Parse(name string, data []byte) (Bundle, error) {
 	b.Date = asTime(b.Extra["date"])
 	b.Tags = terms(b.Extra["tags"])
 	delete(b.Extra, "tags")
+	b.Order = asInt(b.Extra["order"])
+	delete(b.Extra, "order")
 	return b, nil
 }
 
@@ -164,6 +170,20 @@ func asTime(v any) time.Time {
 	return time.Time{}
 }
 
+// asInt reads a frontmatter integer. yaml.v3 hands back an int for an unquoted number and a string for a
+// quoted one, so both spellings work and anything else is simply absent.
+func asInt(v any) int {
+	switch t := v.(type) {
+	case int:
+		return t
+	case string:
+		if n, err := strconv.Atoi(t); err == nil {
+			return n
+		}
+	}
+	return 0
+}
+
 // terms reads a scalar or sequence of tag names, preserving case and script.
 func terms(v any) []string {
 	var out []string