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/render/templates/page.html

@@ -3,4 +3,22 @@
 {{if .Title}}<h1>{{.Title}}</h1>{{end}}
 {{.HTML}}
 </article>
+{{- with .Sequence}}
+{{- if .Index}}
+<nav class="sequence">
+{{- with .Prev}}<a rel="prev" href="{{.URL}}">{{if .Title}}{{.Title}}{{else}}{{.Key}}{{end}}</a>{{end}}
+<span><a href="{{.URL}}">{{if .Title}}{{.Title}}{{else}}{{.URL}}{{end}}</a> {{.Index}} of {{.Count}}</span>
+{{- with .Next}}<a rel="next" href="{{.URL}}">{{if .Title}}{{.Title}}{{else}}{{.Key}}{{end}}</a>{{end}}
+</nav>
+{{- else if .Members}}
+<nav class="sequence">
+<ol class="archive">
+{{- range .Members}}
+<li><a href="{{.URL}}">{{if .Title}}{{.Title}}{{else}}{{.Key}}{{end}}</a>
+{{- if not .Date.IsZero}} <time datetime="{{.Date.Format "2006-01-02"}}">{{.Date.Format "2 January 2006"}}</time>{{end}}</li>
+{{- end}}
+</ol>
+</nav>
+{{- end}}
+{{- end}}
 {{- end}}