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/web/web.go

@@ -138,7 +138,13 @@ func serve(w http.ResponseWriter, req *http.Request, site *content.Site, r *rend
 		http.NotFound(w, req)
 		return
 	}
-	out, err := r.Bundle(b, served, site.Variants(res.key))
+	// A bundle nested under another, or holding others, is part of a series; anything else renders with no
+	// sequence at all (ADR-0033).
+	var seq *content.Sequence
+	if series, inSeries := site.Sequence(b.Key, served); inSeries {
+		seq = &series
+	}
+	out, err := r.Bundle(b, served, site.Variants(res.key), seq)
 	if err != nil {
 		// A render failure degrades: log it and say nothing more to the client than that it failed
 		// (conventions.md). It must never leak a template or filesystem detail.