serve a declared slug as an address, leaving the key alone

The human chose the second option: a route sits beside the key rather than
replacing it. So `slug` renames what a bundle is served at, in every language, and
identity stays derived from the path — which is exactly what keeps ADR-0033 intact,
since series membership is the directory. A series landing page can now be renamed
without orphaning its chapters, and there is a test that says so.

`Site` resolves routes at index time, because only it can see whether every variant
agrees. Disagreement is dropped rather than resolved, as is a slug landing where
another bundle already answers — the same rule colliding keys and contested aliases
already follow. The key a slug moved away from stops answering, so the old address
does not quietly keep working.

Two bugs surfaced doing this, both older than this change:

An alias naming its own bundle's former key was rejected as "an alias that names a
real bundle" — which made rename-plus-alias, the entire point of ADR-0008's alias
mechanism, impossible. The check now asks what a request asks: is anything actually
served there.

Aliases were counted per declaring *file*, so a bundle whose two language variants
both listed the same alias looked like two rival claimants and lost the alias. It is
a set of keys now. This one only appears with translated content, which is why no
fixture had caught it since entry 4 — the real binary did, on the first multilingual
rename.

internal/web/discover.go

@@ -52,7 +52,7 @@ func serveSitemap(w http.ResponseWriter, req *http.Request, site *content.Site,
 	out.WriteString(`<?xml version="1.0" encoding="utf-8"?>` + "\n")
 	out.WriteString(`<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">` + "\n")
 	for _, entry := range site.Everything() {
-		fmt.Fprintf(&out, "<url><loc>%s</loc>", xmlEscape(content.Absolute(base, content.URL(entry.Key, entry.Lang))))
+		fmt.Fprintf(&out, "<url><loc>%s</loc>", xmlEscape(content.Absolute(base, content.URL(entry.Route, entry.Lang))))
 		if !entry.Date.IsZero() {
 			fmt.Fprintf(&out, "<lastmod>%s</lastmod>", entry.Date.Format("2006-01-02"))
 		}