finish the reference theme, and the contract holes it found

The theme was supposed to need no engine work. It needed four things, which is
exactly what the audit said would happen if the contract had gaps rather than the
theme (ADR-0046) — a reader could not reach, from any page: another section, the
tags on the page they were reading, the extras beside it, or the same page in
another language.

So the contract grew three fields, all additive: `.Sections` for navigation,
`.Tags` with each term's listing URL, and `.ExtrasURL`, empty when a bundle has
none so a theme never links a 404. Sections arrive through `Renderer.Navigation`, a
callback, because sections change when content does and a copy would go stale — the
nav updates on a rebuild along with everything else.

One flaw only visible by looking at a rendered page: the language switcher pointed
at the canonical host, because `.Alternates` went absolute for hreflang. Those are
two needs, so an Alternate now carries `.URL` (absolute, for machines) and `.Path`
(relative, for a link a person clicks).

The theme now demonstrates every field it is given, including `.First`/`.Last`,
which existed and were never rendered. Still no JavaScript, still one stylesheet.

A test that asserted a page had no sequence nav was matching the inlined
stylesheet rather than the markup, and now matches the element. That is the third
time a loose assertion has passed for the wrong reason.

internal/web/web_test.go

@@ -387,7 +387,9 @@ func TestPagesOutsideASeriesGetNoSequence(t *testing.T) {
 	if rec.Code != http.StatusOK {
 		t.Fatalf("got %d, want 200", rec.Code)
 	}
-	if body := rec.Body.String(); strings.Contains(body, "sequence") {
+	// Matched on the element, not the word: the stylesheet is inlined into every page, so "sequence" appears in
+	// the CSS whether or not the nav does.
+	if body := rec.Body.String(); strings.Contains(body, `<nav class="sequence">`) {
 		t.Errorf("an unrelated page must render no sequence nav:\n%s", body)
 	}
 }