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/content/site_test.go

@@ -65,6 +65,16 @@ func TestURLPrefixesOnlyNonDefaultLanguages(t *testing.T) {
 			t.Errorf("URL(pages/about, %q) = %q, want %q", lang, got, want)
 		}
 	}
+	// The site root is "/", not "//" — what a whole-site feed asks for, and what a doubled slash would put in
+	// front of every reader as a broken identity.
+	for lang, want := range map[string]string{"en": "/", "": "/", "bn": "/bn/"} {
+		if got := URL("", lang); got != want {
+			t.Errorf("URL(\"\", %q) = %q, want %q", lang, got, want)
+		}
+	}
+	if got := PageURL("", "en", 2); got != "/page/2/" {
+		t.Errorf("PageURL at the root = %q", got)
+	}
 }
 
 func TestAliasesAreParsedAndIndexed(t *testing.T) {