add tag listings, global and section-narrowed

One global namespace (ADR-0018): /tags/{term}/ spans every section and
/{section}/tags/{term}/ narrows it. Listings group by section so one busy term
stays readable, which needed List.Groups alongside Items — list.html renders
whichever is set.

This is Query's second use, so it gained a Tag field rather than being generalised
on speculation: one filter, two callers. Tag slugs lowercase and hyphenate,
preserving script, so "Long Monsoon" and "long monsoon" are one term while Bengali
passes through unchanged. Hand-chosen slugs per term still wait for the type
declaration that owns overrides.

`tags` is reserved at the top level and inside every section, alongside `page` and
the language prefixes. A tag listing redirects to its canonical URL only once it is
known to exist, matching the rule bundles already followed — otherwise a canonical
URL for nothing confirms what is not there.

One stale test expectation fixed rather than worked around: it asserted tags land
in Extra, which stopped being true when tags became a named field.

Evidence: /tags/monsoon/ lists Hello World under posts and First Rain under comics;
/comics/tags/monsoon/ shows one; /tags/monsoon 301s; /tags/nothing/ and /tags/ 404.

internal/render/render.go

@@ -57,6 +57,15 @@ type List struct {
 	Page, Pages int
 	// PrevURL and NextURL are empty at the ends. Newer is "prev" because the order is newest first.
 	PrevURL, NextURL string
+	// Groups is set instead of Items when entries are grouped — a tag listing groups by section, so one
+	// busy term stays readable (ADR-0018).
+	Groups []Group
+}
+
+// Group is a named run of entries within a listing.
+type Group struct {
+	Name  string
+	Items []Item
 }
 
 // Item is one entry in a listing.
@@ -171,6 +180,42 @@ func (r *Renderer) Bundle(b content.Bundle, served string, variants []string) ([
 
 // Listing renders one page of a Query result for a section.
 func (r *Renderer) Listing(section, lang string, all []content.Bundle, page int) ([]byte, error) {
+	l, window := r.paginate(section, lang, content.PageURL(section, lang, page), all, page,
+		func(p int) string { return content.PageURL(section, lang, p) })
+	for _, b := range window {
+		l.Items = append(l.Items, r.item(b, lang))
+	}
+	return r.execute(r.list, l, section)
+}
+
+// Tag renders one page of a tag listing, grouped by section.
+//
+// section narrows the listing to one section and is empty for the global one.
+func (r *Renderer) Tag(section, slug, lang string, all []content.Bundle, page int) ([]byte, error) {
+	title := "#" + slug
+	if section != "" {
+		title = section + " · #" + slug
+	}
+	l, window := r.paginate(title, lang, content.TagURL(section, slug, lang, page), all, page,
+		func(p int) string { return content.TagURL(section, slug, lang, p) })
+	for _, b := range window {
+		sec := b.Section()
+		if n := len(l.Groups); n > 0 && l.Groups[n-1].Name == sec {
+			l.Groups[n-1].Items = append(l.Groups[n-1].Items, r.item(b, lang))
+			continue
+		}
+		l.Groups = append(l.Groups, Group{Name: sec, Items: []Item{r.item(b, lang)}})
+	}
+	return r.execute(r.list, l, "tag "+slug)
+}
+
+// item is one listing entry.
+func (r *Renderer) item(b content.Bundle, lang string) Item {
+	return Item{Title: b.Title, Key: b.Key, URL: content.URL(b.Key, lang), Date: b.Date}
+}
+
+// paginate builds the shell of a listing page and returns the slice of entries it shows.
+func (r *Renderer) paginate(title, lang, canonical string, all []content.Bundle, page int, url func(int) string) (List, []content.Bundle) {
 	pages := (len(all) + content.PerPage - 1) / content.PerPage
 	if pages < 1 {
 		pages = 1
@@ -178,25 +223,17 @@ func (r *Renderer) Listing(section, lang string, all []content.Bundle, page int)
 	start := (page - 1) * content.PerPage
 	end := min(start+content.PerPage, len(all))
 	l := List{
-		head: head{
-			Title:     section,
-			Lang:      lang,
-			Canonical: content.PageURL(section, lang, page),
-			Style:     r.style,
-		},
+		head:  head{Title: title, Lang: lang, Canonical: canonical, Style: r.style},
 		Page:  page,
 		Pages: pages,
 	}
-	for _, b := range all[start:end] {
-		l.Items = append(l.Items, Item{Title: b.Title, Key: b.Key, URL: content.URL(b.Key, lang), Date: b.Date})
-	}
 	if page > 1 {
-		l.PrevURL = content.PageURL(section, lang, page-1)
+		l.PrevURL = url(page - 1)
 	}
 	if page < pages {
-		l.NextURL = content.PageURL(section, lang, page+1)
+		l.NextURL = url(page + 1)
 	}
-	return r.execute(r.list, l, section)
+	return l, all[start:end]
 }
 
 // execute runs a template set and wraps a failure with what was being rendered.

internal/render/templates/list.html

@@ -1,6 +1,16 @@
 {{define "main" -}}
 <h1>{{.Title}}</h1>
-{{if .Items -}}
+{{if .Groups -}}
+{{- range .Groups}}
+<h2>{{.Name}}</h2>
+<ul class="listing">
+{{- range .Items}}
+<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}}
+</ul>
+{{- end}}
+{{- else if .Items -}}
 <ul class="listing">
 {{- range .Items}}
 <li><a href="{{.URL}}">{{if .Title}}{{.Title}}{{else}}{{.Key}}{{end}}</a>