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/web/resolve.go

@@ -14,6 +14,9 @@ type resolution struct {
 	lang string
 	// page is 1 for a bundle or the first listing page, higher for /page/N/.
 	page int
+	// tag is a tag slug when the path named a tag listing; key then holds the section, or "" for the
+	// global listing.
+	tag string
 	// redirect is the canonical path when the request named a non-canonical one. Non-empty means answer
 	// with a permanent redirect and nothing else.
 	redirect string
@@ -60,12 +63,35 @@ func resolve(path string, site *content.Site) (resolution, bool) {
 		}
 		key, page = rest, n
 	}
+	// tags/<term> at the top level, or <section>/tags/<term> inside one (ADR-0018). Recognised by shape
+	// here, so URL knowledge stays in the resolver.
+	tag, section, isTag := cutTag(key)
+
 	if !strings.HasSuffix(path, "/") {
+		if isTag {
+			return resolution{key: section, tag: tag, lang: lang, page: page,
+				redirect: content.TagURL(section, tag, lang, page)}, true
+		}
 		return resolution{key: key, lang: lang, page: page, redirect: content.PageURL(key, lang, page)}, true
 	}
+	if isTag {
+		return resolution{key: section, tag: tag, lang: lang, page: page}, true
+	}
 	return resolution{key: key, lang: lang, page: page}, true
 }
 
+// cutTag splits a tag listing key into its term and the section it is narrowed to.
+func cutTag(key string) (tag, section string, ok bool) {
+	if rest, found := strings.CutPrefix(key, content.TagsSegment+"/"); found {
+		return rest, "", rest != ""
+	}
+	if i := strings.Index(key, "/"+content.TagsSegment+"/"); i >= 0 {
+		term := key[i+len(content.TagsSegment)+2:]
+		return term, key[:i], term != ""
+	}
+	return "", "", false
+}
+
 // cutPage strips a trailing "page/N" off a key, reporting the page number.
 func cutPage(key string) (rest string, page int, ok bool) {
 	i := strings.LastIndex(key, "/")