serve Atom feeds for the site, a section and a tag

Membership is a publication date, not a type declaration (ADR-0043). The parked
feed shape said "every type declared primary", which would have made feeds wait on
declared types a third time — but the thing that distinguishes a feed item is
already on disk. Pages and section landings drop out because they have no date,
which is the right reason. The parked idea stays parked with a sharper trigger:
someone wanting a *dated* bundle kept out.

Built with encoding/xml from typed structs, never a template: XML in html/template
is escaping for the wrong grammar, and that is a correctness trap rather than a
matter of taste.

A bug the evidence found, older than feeds: content.URL("", lang) built "//", so a
whole-site feed's id and alternate link were https://khosra.example// — every
entry identity wrong in every reader. The root is "/" now, with a test, and the
hand-built "/" the resolver carried for the same reason can follow later.

Two counters re-scoped rather than incremented, the same way transforms was:

Views now counts *per-bundle selection* — the thing architecture.md means by the
View layer, still at zero consumers. Output formats are not it: HTML, sitemap XML
and Atom are three functions with nothing to share, so an interface over them
would have one member and no leverage.

Effects stays at 1. A feed is generated per request like the sitemap, so it is not
a second Effect and the runner is not yet due — the next thing that writes files
off the request path is.

internal/web/web.go

@@ -18,7 +18,7 @@ import (
 func Handler(site *content.Site, r *render.Renderer, siteFS, derivedFS fs.FS, settings content.Settings) http.Handler {
 	mux := http.NewServeMux()
 	mux.HandleFunc("GET /", func(w http.ResponseWriter, req *http.Request) {
-		serve(w, req, site, r, siteFS)
+		serve(w, req, site, r, siteFS, settings)
 	})
 	// Two exact paths a crawler asks for by name, so they are mux entries rather than resolver cases: no
 	// bundle can own them, since a key always sits under a section.
@@ -128,12 +128,18 @@ func writeAs(w http.ResponseWriter, contentType string, out []byte, what string)
 }
 
 // serve resolves one request and writes its bundle.
-func serve(w http.ResponseWriter, req *http.Request, site *content.Site, r *render.Renderer, siteFS fs.FS) {
+func serve(w http.ResponseWriter, req *http.Request, site *content.Site, r *render.Renderer, siteFS fs.FS, settings content.Settings) {
 	res, ok := resolve(req.URL.Path, site)
 	if !ok {
 		http.NotFound(w, req)
 		return
 	}
+	if res.feed {
+		if !serveFeed(w, req, site, res, settings) {
+			http.NotFound(w, req)
+		}
+		return
+	}
 	if res.tag != "" {
 		if !serveTags(w, req, site, r, res) {
 			http.NotFound(w, req)